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HYPK (UniProt accession: Q9NX55) encodes Huntingtin-interacting protein K, also known as huntingtin yeast partner K, in humans (Homo sapiens). The gene is located on chromosome 15 (synonyms: C15orf63, HSPC136) and belongs to a conserved family of regulatory proteins involved in co-translational protein processing and proteostasis (liu2023huntingtininteractingproteins pages 1-2, liu2023huntingtininteractingproteins pages 2-4).
HYPK exhibits dual, mechanistically distinct molecular functions that have been elucidated through recent structural and biochemical studies:
HYPK functions as a non-catalytic regulatory subunit of the NatA complex, which is responsible for N-terminal acetylation of up to 40-50% of mammalian proteins (deng2021proteinnterminalacetylation pages 2-3, damme2021chartingthenterminal pages 1-2). The NatA complex comprises the catalytic subunit NAA10 and the large ribosome-anchoring auxiliary subunit NAA15. HYPK physically associates with this NatA heterodimer and exerts two key regulatory effects:
Stabilization: HYPK binding has a strong stabilizing effect on the NatA complex, promoting proper assembly and maintenance of the NAA10-NAA15 heterodimer (deng2021proteinnterminalacetylation pages 2-3, klein2024multiproteinassembliesorchestrate pages 2-3).
Catalytic Inhibition: HYPK inhibits the catalytic activity of NAA10 through an allosteric mechanism. Structural studies demonstrate that HYPK binding to the NAA15 α-helical scaffold induces conformational rearrangements in the first two tetratricopeptide repeat (TPR) motifs of NAA15. This bending occurs in the opposite direction compared to ribosome-bound NatA, indicating structural plasticity (klein2024multiproteinassembliesorchestrate pages 2-3). These rearrangements allosterically inhibit NAA10 and can also prevent NAA50 (a component of the NatE complex) from binding to NatA (deng2021proteinnterminalacetylation pages 2-3, damme2021chartingthenterminal pages 1-2).
Coordination of Co-Translational Processing: Recent cryo-EM structures from 2024 reveal that HYPK participates in organizing multi-protein assemblies at the ribosome exit tunnel for coordinated nascent chain processing (klein2024multiproteinassembliesorchestrate pages 1-2, lentzsch2024nacguidesa pages 1-9). HYPK can be detected in ribosomal complexes that include NatA, the nascent polypeptide-associated complex (NAC), and methionine aminopeptidases (MAP1 or MAP2), supporting a model in which HYPK helps tune the assembly states of co-translational processing machinery (klein2024multiproteinassembliesorchestrate pages 1-2, klein2024multiproteinassembliesorchestrate pages 2-3).
It is important to note that HYPK is a regulatory protein, not an enzyme with intrinsic substrate specificity. Rather, it modulates the activity of NatA, which acetylates protein N-termini starting with small, uncharged amino acids (Ser, Ala, Gly, Thr, Val, Cys) after initiator methionine removal (damme2021chartingthenterminal pages 1-2).
In a functionally independent role, HYPK serves as an autophagy receptor that mediates the selective degradation of polyneddylated proteins during proteotoxic stress (ghosh2022hypkcoordinatesdegradation pages 1-2, ghosh2022hypkcoordinatesdegradation pages 2-4).
NEDD8 Recognition: HYPK binds to the ubiquitin-like protein NEDD8 through its C-terminal ubiquitin-associated (UBA) domain. Biochemical analyses including surface plasmon resonance and immunoprecipitation demonstrate that HYPK binds both monomeric NEDD8 and polyneddylated chains formed on misfolded or aggregated proteins (ghosh2022hypkcoordinatesdegradation pages 6-8, ghosh2022hypkcoordinatesdegradation pages 8-10). Critical conserved residues in the UBA domain—aspartate-94 (D94), glutamate-101 (E101), leucine-113 (L113), and glycine-118 (G118)—are required for efficient NEDD8 binding, with E101 and G118 positioned for direct interaction and D94 and L113 stabilizing the UBA fold (ghosh2022hypkcoordinatesdegradation pages 8-10).
LC3/GABARAP Interaction: HYPK contains an atypical tyrosine-type LC3-interacting region (LIR) motif with the sequence Y49AEE52 in its N-terminal region (ghosh2022hypkcoordinatesdegradation pages 10-11, ghosh2022hypkcoordinatesdegradation pages 8-10). This motif differs from canonical LIR sequences by having an acidic amino acid (glutamate) at the fourth position instead of a hydrophobic residue. Mutational analysis and binding assays confirm that this region mediates direct interaction with LC3A, LC3B, GABARAP, GABARAPL1, and GABARAPL2—members of the ATG8 family that decorate autophagosomal membranes (ghosh2022hypkcoordinatesdegradation pages 10-11).
Scaffolding Function in Aggrephagy: By simultaneously binding NEDD8-modified cargo through its UBA domain and autophagosomal LC3/GABARAP proteins through its LIR, HYPK functions as a molecular scaffold that bridges polyneddylated protein aggregates to the autophagy machinery (ghosh2022hypkcoordinatesdegradation pages 1-2, ghosh2022hypkcoordinatesdegradation pages 10-11, ghosh2022hypkcoordinatesdegradation pages 8-10). This scaffolding activity is analogous to the well-characterized ubiquitin-LC3 bridging performed by SQSTM1/p62 in ubiquitin-dependent autophagy, but operates in a NEDD8-specific pathway.
Autophagy Modulation: Beyond cargo recognition, HYPK positively modulates autophagy flux. HYPK depletion reduces LC3B lipidation, autophagosome formation, and autolysosome maturation, whereas HYPK overexpression increases these parameters (ghosh2022hypkcoordinatesdegradation pages 10-11). HYPK knockdown also reduces phosphorylation of BECN1 at serine-15, a key autophagy initiation signal (ghosh2022hypkcoordinatesdegradation pages 2-4). Thus, HYPK not only recruits cargo but also promotes autophagy induction and maturation.
HYPK possesses a tripartite domain architecture that supports its dual functions (deng2021proteinnterminalacetylation pages 2-3, ghosh2022hypkcoordinatesdegradation pages 10-11, ghosh2022hypkcoordinatesdegradation pages 8-10):
N-Terminal Region: An extended, largely disordered N-terminus containing the atypical Y-type LIR motif (Y49AEE52) for LC3/GABARAP binding. This region also harbors hydrophobic patches and low-complexity regions that contribute to HYPK self-oligomerization during stress (ghosh2022hypkcoordinatesdegradation pages 10-11, ghosh2022hypkcoordinatesdegradation pages 8-10).
Central Long α-Helix: A long α-helical region that connects the N-terminal and C-terminal domains and likely contributes to the overall structural organization of HYPK (deng2021proteinnterminalacetylation pages 2-3).
C-Terminal UBA Domain: A three-helix bundle UBA domain (~45 amino acids) homologous to the C-terminus of NACα and structurally similar to the second UBA domain of NUB1 (klein2024multiproteinassembliesorchestrate pages 2-3, ghosh2022hypkcoordinatesdegradation pages 8-10). This domain mediates interactions with NEDD8, ubiquitin-like modifiers, and the NatA complex. Key conserved residues D94, E101, L113, and G118 are critical for NEDD8 binding (ghosh2022hypkcoordinatesdegradation pages 8-10).
The domain architecture of HYPK is consistent with the UniProt annotations indicating Huntingtin-int_K, HYPK_UBA, and NAC-like_UBA domains, and is supported by crystal structures and cryo-EM reconstructions (lentzsch2024nacguidesa pages 1-9, klein2024multiproteinassembliesorchestrate pages 2-3, ghosh2022hypkcoordinatesdegradation pages 8-10).
HYPK exhibits dynamic subcellular localization that reflects its dual roles:
Ribosome-Associated Localization: HYPK localizes to polysomes and has been reported to co-localize with ribosome-associated factors (klein2024multiproteinassembliesorchestrate pages 2-3). Cryo-EM structures from 2024 reveal HYPK in complex with NatA on the surface of translating 80S ribosomes, positioned near the polypeptide exit tunnel in coordination with NAC and methionine aminopeptidases (klein2024multiproteinassembliesorchestrate pages 1-2, lentzsch2024nacguidesa pages 1-9, klein2024multiproteinassembliesorchestrate pages 2-3). This ribosomal localization supports HYPK's role in regulating co-translational N-terminal acetylation.
Cytoplasmic Stress Granule Formation: During proteotoxic stress (e.g., induced by puromycin treatment), HYPK forms cytoplasmic foci or granules that colocalize with NEDD8-modified protein aggregates and LC3-positive autophagosomal structures (ghosh2022hypkcoordinatesdegradation pages 6-8, ghosh2022hypkcoordinatesdegradation pages 4-6, ghosh2022hypkcoordinatesdegradation pages 10-11, ghosh2022hypkcoordinatesdegradation pages 8-10). This relocalization is neddylation-dependent: NEDD8 knockdown prevents HYPK granule formation (ghosh2022hypkcoordinatesdegradation pages 8-10). The granules represent sites of HYPK-mediated aggrephagy, where polyneddylated aggregates are sequestered and delivered to autophagosomes for degradation.
HYPK participates in the coordinated processing of nascent polypeptides emerging from the ribosome. This pathway involves sequential enzymatic steps:
N-Terminal Methionine Excision (NME): Methionine aminopeptidases (MAP1 and MAP2) cleave the initiator methionine from nascent chains when the second residue is small and uncharged (klein2024multiproteinassembliesorchestrate pages 1-2, klein2024multiproteinassembliesorchestrate pages 2-3).
N-Terminal Acetylation (NTA): The NatA complex (NAA10-NAA15) acetylates the newly exposed N-terminus (klein2024multiproteinassembliesorchestrate pages 1-2, klein2024multiproteinassembliesorchestrate pages 2-3, damme2021chartingthenterminal pages 1-2).
HYPK regulates this process through its interactions with NatA and coordination with NAC. Structural studies show that NatA can occupy a non-intrusive "distal" ribosomal binding site compatible with concurrent MAP1 or MAP2 binding, allowing both enzymatic activities to operate in close proximity (klein2024multiproteinassembliesorchestrate pages 1-2, klein2024multiproteinassembliesorchestrate pages 2-3). NAC facilitates MAP1 recruitment and helps organize a dynamic multi-enzyme assembly that includes HYPK-NatA (klein2024multiproteinassembliesorchestrate pages 1-2, klein2024multiproteinassembliesorchestrate pages 2-3). HYPK's inhibitory activity on NatA may provide temporal control, ensuring that acetylation occurs only after appropriate substrate exposure and MAP activity (deng2021proteinnterminalacetylation pages 2-3, klein2024multiproteinassembliesorchestrate pages 2-3).
Dysregulation of NatA function has been linked to congenital heart disease and neurodevelopmental disorders, highlighting the importance of HYPK as a regulatory component (ward2021mechanismsofcongenital pages 1-2).
HYPK mediates a specialized autophagy pathway for polyneddylated protein aggregates:
Proteotoxic Stress and Neddylation: Under proteotoxic conditions (e.g., translation inhibition, misfolding stress), certain misfolded proteins become modified with NEDD8 chains. This polyneddylation marks aggregation-prone substrates, including mutant huntingtin exon 1 (HTT97Q exon 1) (ghosh2022hypkcoordinatesdegradation pages 1-2, ghosh2022hypkcoordinatesdegradation pages 6-8, ghosh2022hypkcoordinatesdegradation pages 4-6).
HYPK-Mediated Recognition and Sequestration: HYPK recognizes polyneddylated aggregates via its UBA domain and self-oligomerizes to form annular structures that sequester heterogeneous protein aggregates (ghosh2022hypkcoordinatesdegradation pages 8-10). These HYPK-positive granules colocalize with NEDD8 and LC3, forming sites of autophagosomal cargo capture (ghosh2022hypkcoordinatesdegradation pages 6-8, ghosh2022hypkcoordinatesdegradation pages 10-11, ghosh2022hypkcoordinatesdegradation pages 8-10).
Autophagosome Formation and Maturation: HYPK's LIR-mediated interaction with LC3/GABARAP recruits autophagosomal membranes to the neddylated cargo. HYPK also promotes autophagy initiation by enhancing BECN1 phosphorylation and LC3 lipidation (ghosh2022hypkcoordinatesdegradation pages 2-4, ghosh2022hypkcoordinatesdegradation pages 10-11). Autophagosomes mature into autolysosomes for degradation of the sequestered aggregates.
Cytoprotection: This pathway protects cells from proteotoxic damage. HYPK or NEDD8 depletion impairs aggregate clearance, leading to accumulation of toxic aggregates such as mutant HTT exon 1, whereas HYPK or NEDD8 overexpression enhances clearance (ghosh2022hypkcoordinatesdegradation pages 1-2, ghosh2022hypkcoordinatesdegradation pages 6-8, ghosh2022hypkcoordinatesdegradation pages 4-6).
This NEDD8-HYPK-LC3 axis represents a noncanonical autophagy pathway distinct from the well-known ubiquitin-SQSTM1-LC3 pathway. SQSTM1 levels remain unchanged during HYPK-mediated autophagy, indicating pathway independence (ghosh2022hypkcoordinatesdegradation pages 2-4, ghosh2022hypkcoordinatesdegradation pages 8-10).
HYPK was originally identified as a huntingtin-interacting protein (liu2023huntingtininteractingproteins pages 1-2, liu2023huntingtininteractingproteins pages 2-4). Its name reflects this discovery, and recent work has mechanistically connected this identity to aggregate clearance. Mutant huntingtin with expanded polyglutamine tracts forms toxic aggregates that are polyneddylated and recognized by HYPK for autophagic degradation (ghosh2022hypkcoordinatesdegradation pages 1-2, ghosh2022hypkcoordinatesdegradation pages 6-8). Thus, HYPK links its original huntingtin-interacting function to a defined role in Huntington's disease-relevant proteostasis (ghosh2022hypkcoordinatesdegradation pages 1-2, liu2023huntingtininteractingproteins pages 2-4).
The NatA complex is essential for development, and haploinsufficiency of NAA15 (the NatA auxiliary subunit with which HYPK interacts) causes congenital heart disease and neurodevelopmental deficits (ward2021mechanismsofcongenital pages 1-2). HYPK's regulatory role in NatA function suggests that perturbations in HYPK expression or activity could contribute to similar disorders. Additionally, HYPK has been implicated in regulating autophagy and protein folding responses, and its interaction with proteins involved in cell cycle arrest and unfolded protein response underscores broader roles in cellular homeostasis (pozoga2022fromnucleusto pages 1-2).
HYPK functions as a proteostasis factor during stress. Its ability to form stress granules, recognize neddylated aggregates, and promote autophagy positions HYPK as a key player in the cellular response to proteotoxic insults (ghosh2022hypkcoordinatesdegradation pages 1-2, ghosh2022hypkcoordinatesdegradation pages 2-4, ghosh2022hypkcoordinatesdegradation pages 10-11, ghosh2022hypkcoordinatesdegradation pages 8-10). This role has implications for aging, neurodegeneration, and cancer, where protein quality control is frequently compromised.
Recent structural biology advances have dramatically refined our understanding of HYPK:
2024 Cryo-EM Studies: High-resolution cryo-EM structures of human NatA-ribosome complexes, including ternary assemblies with MAP2 or NAC-MAP1, have revealed the precise positioning and conformational dynamics of HYPK-associated NatA during co-translational processing (klein2024multiproteinassembliesorchestrate pages 1-2, lentzsch2024nacguidesa pages 1-9). These studies show that HYPK induces rearrangements in NAA15 that are distinct from but complementary to ribosome binding, supporting a model in which HYPK fine-tunes NatA assembly states (klein2024multiproteinassembliesorchestrate pages 2-3).
2023 Reviews on Huntingtin-Interacting Proteins: Comprehensive reviews have integrated HYPK into the broader network of huntingtin-interacting proteins, highlighting its role in both NatA regulation and autophagy as dual mechanisms by which HYPK may influence Huntington's disease pathogenesis (liu2023huntingtininteractingproteins pages 1-2, liu2023huntingtininteractingproteins pages 2-4).
2024 Studies on NAT Regulation in Plants: Comparative studies in photosynthetic organisms have shown that HYPK homologs also regulate NatA in plants, where they influence stress responses and protein stability, suggesting evolutionary conservation of HYPK's regulatory functions (pozoga2022fromnucleusto pages 1-2, giglione2021evolutiondrivenversatilityof pages 1-6).
2024 Influenza and Acetylation: HYPK was identified as part of a regulatory network involving the NatA complex that affects influenza A virus replication, underscoring roles for HYPK in host-pathogen interactions mediated through acetylation pathways (klein2024multiproteinassembliesorchestrate pages 1-2).
| Function/Role | Molecular Mechanism | Key Domains/Motifs | Binding Partners | Subcellular Localization | Biological Processes |
|---|---|---|---|---|---|
| NatA regulatory subunit / co-translational processing factor | HYPK physically associates with the human NatA complex (NAA10-NAA15), has a strong stabilizing effect on the complex, and inhibits NAA10 catalytic activity. Structural studies indicate HYPK binding to the NAA15 helical scaffold induces conformational rearrangements that can allosterically hinder NAA50 association, helping regulate the composition and activity of ribosome-bound N-terminal acetylation machinery. HYPK is therefore a regulator, not a catalytic enzyme itself. (deng2021proteinnterminalacetylation pages 2-3, klein2024multiproteinassembliesorchestrate pages 2-3, deng2021proteinnterminalacetylation pages 3-5) | C-terminal three-helix bundle UBA domain; central long α-helix; extended/disordered N-terminus; Huntingtin-int_K/HYPK_UBA-compatible architecture from UniProt is consistent with the NatA-bound structural literature. (klein2024multiproteinassembliesorchestrate pages 2-3, deng2021proteinnterminalacetylation pages 3-5) | NAA10, NAA15, indirectly antagonizes NAA50 association with NatA; functionally linked to NAC and MAP1/MAP2 assemblies on the ribosome. (klein2024multiproteinassembliesorchestrate pages 2-3, damme2021chartingthenterminal pages 1-2) | Ribosome-associated; polysome-associated; positioned near the ribosomal polypeptide tunnel exit together with NatA and NAC during nascent-chain processing. (klein2024multiproteinassembliesorchestrate pages 2-3, lentzsch2024nacguidesa pages 1-9) | Co-translational N-terminal acetylation; coordination of N-terminal methionine excision and N-terminal acetylation; early protein biogenesis and proteostasis. (klein2024multiproteinassembliesorchestrate pages 1-2, klein2024multiproteinassembliesorchestrate pages 2-3, damme2021chartingthenterminal pages 1-2) |
| Organizer of ribosome-proximal multi-enzyme assemblies | Recent cryo-EM work places NatA in a distal ribosomal site compatible with co-occupancy by MAP2 or by NAC-MAP1. HYPK-associated structural rearrangements in NAA15 resemble part of the plasticity required for ribosome engagement, supporting a model in which HYPK helps tune assembly states of the co-translational processing apparatus rather than acting as a substrate-binding enzyme. (klein2024multiproteinassembliesorchestrate pages 2-3) | Interface-forming helical architecture; flexible/disordered N-terminus likely supports dynamic assembly; C-terminal UBA domain homologous in part to NACα C-terminus per recent discussion. (klein2024multiproteinassembliesorchestrate pages 2-3) | NatA complex, NAC, MAP1/MAP2-linked ribosome processing environment. (klein2024multiproteinassembliesorchestrate pages 2-3) | 80S ribosome surface at/near the polypeptide exit tunnel. (klein2024multiproteinassembliesorchestrate pages 1-2, klein2024multiproteinassembliesorchestrate pages 2-3) | Dynamic assembly of ribosome-associated factors; coordination of sequential processing of nascent polypeptides. (klein2024multiproteinassembliesorchestrate pages 1-2, klein2024multiproteinassembliesorchestrate pages 2-3) |
| Selective autophagy receptor for polyneddylated cargo | HYPK acts as a scaffold that links polyneddylated aggregates to the autophagy machinery. It binds NEDD8 through its C-terminal UBA domain and binds LC3/GABARAP proteins through an N-terminal atypical LC3-interacting region, thereby promoting autophagic sequestration and degradation of neddylated protein aggregates. This defines HYPK as an autophagy receptor in NEDD8-dependent aggrephagy. (ghosh2022hypkcoordinatesdegradation pages 1-2, ghosh2022hypkcoordinatesdegradation pages 10-11, ghosh2022hypkcoordinatesdegradation pages 8-10) | C-terminal UBA domain; N-terminal atypical tyrosine-type LIR motif Y49AEE52. (ghosh2022hypkcoordinatesdegradation pages 10-11, ghosh2022hypkcoordinatesdegradation pages 8-10) | NEDD8; LC3A; LC3B; GABARAP; GABARAPL1; GABARAPL2. (ghosh2022hypkcoordinatesdegradation pages 10-11, ghosh2022hypkcoordinatesdegradation pages 8-10) | Cytoplasmic puncta/granules; colocalizes with NEDD8 granules and LC3-positive autophagic structures during proteotoxic stress. (ghosh2022hypkcoordinatesdegradation pages 4-6, ghosh2022hypkcoordinatesdegradation pages 10-11) | Selective autophagy; aggrephagy; proteotoxic stress response; clearance of insoluble protein aggregates. (ghosh2022hypkcoordinatesdegradation pages 1-2, ghosh2022hypkcoordinatesdegradation pages 4-6, ghosh2022hypkcoordinatesdegradation pages 10-11) |
| NEDD8-binding cargo adaptor | HYPK noncovalently binds monomeric NEDD8 and polyneddylated chains. Mutational analysis identified conserved UBA residues D94, E101, L113, and G118 as important for efficient NEDD8 binding; modeling suggests E101 and G118 are exposed for direct interaction whereas D94 and L113 help stabilize the UBA fold. (ghosh2022hypkcoordinatesdegradation pages 8-10) | UBA domain; critical residues D94, E101, L113, G118. (ghosh2022hypkcoordinatesdegradation pages 8-10) | NEDD8; likely recognizes polyneddylated aggregate cargo including mutant HTT exon 1. (ghosh2022hypkcoordinatesdegradation pages 6-8, ghosh2022hypkcoordinatesdegradation pages 8-10) | Cytoplasm, especially stress-induced aggregate-containing regions. (ghosh2022hypkcoordinatesdegradation pages 4-6, ghosh2022hypkcoordinatesdegradation pages 8-10) | Recognition of neddylated aggregates as autophagic cargo; noncanonical NEDD8-mediated protein quality control. (ghosh2022hypkcoordinatesdegradation pages 1-2, ghosh2022hypkcoordinatesdegradation pages 8-10) |
| LC3/GABARAP-binding autophagy scaffold | HYPK contains a noncanonical Y-type LIR in its N-terminal region. Biochemical and cell-based assays show full-length HYPK and its N-terminal fragment bind LC3B, whereas mutation of the Y49/E51/E52 region disrupts this interaction, demonstrating a direct mechanism for autophagosome recruitment. (ghosh2022hypkcoordinatesdegradation pages 10-11) | Atypical LIR motif Y49AEE52 in N-terminal region. (ghosh2022hypkcoordinatesdegradation pages 10-11) | LC3B and other ATG8-family proteins. (ghosh2022hypkcoordinatesdegradation pages 10-11) | Cytoplasmic autophagic membranes/puncta. (ghosh2022hypkcoordinatesdegradation pages 10-11) | Autophagosome formation and maturation; selective autophagy receptor function. (ghosh2022hypkcoordinatesdegradation pages 10-11) |
| Positive modulator of basal and stress-induced autophagy | HYPK depletion lowers LC3B-II levels and LC3 puncta, whereas overexpression increases autophagosomes and supports ongoing autophagic flux and autolysosome formation. Thus HYPK is not only a cargo receptor but also a positive regulator of autophagy efficiency in several human cell systems. (ghosh2022hypkcoordinatesdegradation pages 10-11) | UBA-LIR dual-module scaffold; self-oligomerization-promoting regions contribute to puncta formation. (ghosh2022hypkcoordinatesdegradation pages 10-11, ghosh2022hypkcoordinatesdegradation pages 8-10) | LC3 family proteins; NEDD8-modified cargo; autophagy machinery downstream of cargo recognition. (ghosh2022hypkcoordinatesdegradation pages 10-11, ghosh2022hypkcoordinatesdegradation pages 8-10) | Cytoplasm; autophagosome/autolysosome pathway. (ghosh2022hypkcoordinatesdegradation pages 10-11) | Basal autophagy; stress-induced autophagy; autophagic flux maintenance. (ghosh2022hypkcoordinatesdegradation pages 4-6, ghosh2022hypkcoordinatesdegradation pages 10-11) |
| Proteostasis factor linked to mutant huntingtin aggregate clearance | In cell models, NEDD8 and HYPK promote clearance of mutant HTT exon 1 aggregates by autophagy. HYPK therefore connects its original huntingtin-interacting identity with a mechanistically defined role in aggregate disposal, especially for proteotoxic, aggregation-prone substrates. (ghosh2022hypkcoordinatesdegradation pages 1-2, ghosh2022hypkcoordinatesdegradation pages 6-8, liu2023huntingtininteractingproteins pages 2-4) | Huntingtin-interacting protein architecture with UBA and LIR modules enabling cargo recognition and autophagy coupling. (liu2023huntingtininteractingproteins pages 2-4, ghosh2022hypkcoordinatesdegradation pages 8-10) | Mutant HTT exon 1 aggregates; NEDD8; LC3. (ghosh2022hypkcoordinatesdegradation pages 6-8, ghosh2022hypkcoordinatesdegradation pages 10-11) | Cytoplasmic aggregate foci in neuronal and other cultured cells under proteotoxic stress. (ghosh2022hypkcoordinatesdegradation pages 6-8, ghosh2022hypkcoordinatesdegradation pages 10-11) | Aggregate clearance, cytoprotection during proteotoxic stress, Huntington disease-relevant protein quality control. (ghosh2022hypkcoordinatesdegradation pages 1-2, ghosh2022hypkcoordinatesdegradation pages 6-8, liu2023huntingtininteractingproteins pages 2-4) |
| Context within NatA biology and human disease-related proteostasis | NatA modifies up to ~40-50% of mammalian proteins, primarily iMet-processed N-termini beginning with Ser/Ala/Gly/Thr/Val/Cys; HYPK does not define substrate sequence specificity itself but regulates the major NatA complex that does. Disturbance of NatA biology is linked to developmental disease, underscoring the importance of HYPK as a regulatory component of this pathway. (damme2021chartingthenterminal pages 1-2, ward2021mechanismsofcongenital pages 1-2) | NatA-associated regulatory architecture rather than catalytic GNAT fold. (deng2021proteinnterminalacetylation pages 2-3, damme2021chartingthenterminal pages 1-2) | NAA10, NAA15, functionally NAA50-associated NatE states. (damme2021chartingthenterminal pages 1-2, ward2021mechanismsofcongenital pages 1-2) | Ribosome-associated cytosol. (damme2021chartingthenterminal pages 1-2) | N-terminal acetylome regulation, protein stability control, developmental proteostasis. (damme2021chartingthenterminal pages 1-2, ward2021mechanismsofcongenital pages 1-2) |
Table: This table summarizes the best-supported molecular functions, domains, interactions, localization, and biological roles of human HYPK from the collected literature. It highlights the protein’s dual roles in NatA-mediated co-translational protein processing and NEDD8-dependent selective autophagy.
HYPK (Huntingtin-interacting protein K) is a multifunctional regulatory protein with two principal, mechanistically distinct roles: (1) as a non-catalytic regulatory subunit of the NatA N-terminal acetyltransferase complex, where it stabilizes the complex and allosterically inhibits catalytic activity while coordinating co-translational nascent chain processing; and (2) as a selective autophagy receptor for polyneddylated protein aggregates during proteotoxic stress, bridging NEDD8-modified cargo to LC3/GABARAP-positive autophagosomes.
HYPK localizes to ribosomes for NatA regulation and to cytoplasmic stress granules for aggrephagy. Its domain architecture—comprising an N-terminal LIR-containing disordered region, a central α-helix, and a C-terminal UBA domain—supports these dual scaffolding functions. HYPK does not possess intrinsic enzymatic activity or substrate specificity; rather, it modulates the activity of the NatA complex (which acetylates ~40-50% of the mammalian proteome) and facilitates the degradation of specific NEDD8-modified aggregates.
Recent structural studies from 2024 have provided unprecedented molecular detail on HYPK's integration into ribosomal multi-enzyme assemblies and its regulatory mechanisms. Dysregulation of HYPK-associated pathways has been implicated in developmental disorders, neurodegeneration (particularly Huntington's disease), and proteostasis defects. HYPK thus represents a critical node linking co-translational protein processing and stress-induced protein quality control, with broad implications for cellular homeostasis and disease.
References
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