| Functional role | Key molecular mechanisms | Subcellular localization | Evidence type / key recent citations |
|---|---|---|---|
| Selective autophagy / lysosomal targeting | HTT acts as a scaffold for selective autophagy and lysosomal cargo delivery; a ubiquitin-binding domain (UBD) at residues 235–367 binds ubiquitin/ubiquitinated cargo; HTT interacts with ULK1, p62/SQSTM1, and LC3; HTT knockout alters lysosomal cargo composition, reducing lysosomal targeting of mitochondrial proteins and altering RNA-binding protein delivery | Cytoplasm; autophagosomes; lysosomes/endolysosomes; mitochondria-associated degradative pathway | Primary experimental evidence (biochemistry, CRISPR KO, LysoIP proteomics, imaging) and recent review synthesis (pqac-00000009, pqac-00000010, pqac-00000013, pqac-00000014) |
| Vesicle / motor transport | HTT is a multifunctional scaffold for bidirectional vesicle transport; S421 phosphorylation biases motor engagement, with dephosphorylation favoring dynein-mediated retrograde transport and phosphorylation favoring kinesin-1 recruitment/anterograde transport; key partners include HAP1, kinesin-1, dynein/dynactin; HTT also associates with RAB-positive vesicles and endosomal machinery | Axons and neurites; cytoplasmic vesicles; endosomes; BDNF-containing transport vesicles | Primary experimental evidence and mechanistic review support (pqac-00000009, pqac-00000010, pqac-00000012, pqac-00000016) |
| Autophagosome maturation / retrograde axonal trafficking | HTT cooperates with HAP1 and transport machinery to support retrograde autophagosome movement; HTT and STX17 co-localize on RAB7+ endolysosomes; STX17–RAB7 fusion state is linked to retrograde transport competence; mutant HTT perturbs STX17-mediated fusion and RILP–dynactin interactions | Distal axon; autophagosomes; RAB7+ endolysosomes; lysosome fusion compartments | Review integrating neuronal experimental literature (pqac-00000012, pqac-00000014) |
| Nuclear–cytoplasmic shuttling / transcription-related scaffolding | HTT contains NLS and C-terminal NES elements; the N17 N-terminal amphipathic helix promotes membrane association and limits nuclear accumulation; deletion/disruption of N17 increases nuclear localization; HTT interacts with transcription factors/co-regulators, supporting a scaffold role in transcriptional regulation | Nucleus; cytoplasm; membrane-associated compartments | Structural/functional review evidence and disease-focused review synthesis (pqac-00000011, pqac-00000015, pqac-00000002) |
| Membrane targeting / lipid sensing | N17 forms an amphipathic α-helix important for membrane targeting; HTT shows preferential binding to phosphoinositides including PI3,5P2 and PI4,5P2; palmitoylation at C214 (via HIP14/ZDHHC17 and HIP14L/ZDHHC13) modulates membrane association and trafficking behavior | Plasma membrane; endolysosomal/autophagic membranes; ER/Golgi-associated membranes | Review synthesis grounded in prior structural/cell-biological studies (pqac-00000011, pqac-00000014) |
| Cytoskeleton / actin organization | Beyond microtubule-based transport, HTT directly organizes actin: the N-HEAT and Bridge domains wrap around F-actin, and HTT dimerization can bridge parallel actin filaments; this supports growth-cone morphology and cytoskeletal organization | F-actin networks; axonal growth cones; neuronal cytoskeleton | Recent structural primary study (2025) extending HTT functional annotation beyond trafficking (pqac-00000000) |
| Ciliogenesis / mitotic spindle / cell polarity (brief) | HTT has established roles in ciliogenesis, mitotic spindle positioning, and cell polarity/developmental organization; these functions are frequently interpreted as consistent with its broader scaffold/adaptor role, though mechanistic resolution is less complete than for transport/autophagy | Centrosome / ciliary base; mitotic apparatus; developing neural cells | Disease/developmental review and organoid-model evidence summaries (pqac-00000005, pqac-00000015) |
| Defining structural features for annotation | Large ~3144 aa (~348 kDa) HEAT/ARM-like scaffold protein with N-terminal polyQ tract, N17 amphipathic helix, polyproline-rich region, and resolved HTT–HAP40 architecture; these features support classification as a non-enzymatic scaffold/adaptor rather than enzyme/transporter | Broadly cytoplasmic with dynamic membrane and nuclear association | Authoritative review and structural evidence (pqac-00000002, pqac-00000015) |


*Table: This table summarizes the main experimentally supported functions, mechanisms, and localizations of human HTT/huntingtin for annotation purposes. It prioritizes recent 2023–2024 evidence where available and highlights the scaffold/adaptor nature of HTT across autophagy, trafficking, and compartmental regulation.*