Gene Ontology annotation based on Enzyme Commission mapping
Gene Ontology annotation based on UniPathway vocabulary mapping
Gene Ontology annotation based on UniProtKB/Swiss-Prot keyword mapping
Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular Location vocabulary mapping, accompanied by conservative changes to GO terms applied by UniProt
Electronic Gene Ontology annotations created by ARBA machine learning models
Combined Automated Annotation using Multiple IEA Methods
Downregulation of major histocompatibility complex class I molecules by Kaposi's sarcoma-associated herpesvirus K3 and K5 proteins.
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K3 downregulates all four HLA allotypes tested (HLA-A2, HLA-B2705, HLA-Cw4 and HLA-E), whereas K5 spares HLA-E and acts only weakly on HLA-C; the allotype selectivity maps in part to the transmembrane regions.
"A dramatic downregulation of HLA-A2, HLA-B2705, HLA-Cw4, and HLA-E was observed in the K3-expressing GFP-positive cell population"
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Antibody-internalisation microscopy shows K3 accelerates endocytosis of class I: internalisation is evident within 30 min and nearly complete by 60 min, against roughly 120 min in control cells, while transferrin receptor endocytosis is unchanged.
"K3-expressing cells exhibited a rapid endocytosis of class I molecules"
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Confocal immunofluorescence places K3 near the host plasma membrane, with little or no overlap with a Golgi marker.
"cross sections of transfected cells demonstrated that K3 and K5 were localized near the plasma membrane"
Kaposi's sarcoma-associated herpesvirus encodes two proteins that block cell surface display of MHC class I chains by enhancing their endocytosis.
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K3 and K5 do not retain class I in the ER or block its assembly or export; they accelerate endocytosis from the surface and route internalised chains to endolysosomal vesicles for degradation.
"they act to enhance endocytosis of MHC I from the cell surface; internalized class I chains are delivered to endolysosomal vesicles, where they undergo degradation"
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Colocalisation with KDEL-GFP and with endogenous calnexin, and absence of overlap with later-compartment markers, places the steady-state bulk of K3 in the host endoplasmic reticulum.
"Thus, at steady state the bulk of the K3 and K5 chains are localized within the ER"
A novel class of herpesvirus-encoded membrane-bound E3 ubiquitin ligases regulates endocytosis of proteins involved in immune recognition.
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MIR1 (K3) and MIR2 (K5) expression causes ubiquitination of endogenous MHC class I chains, and the activity requires the N-terminal Zn finger; the paper defines the MARCH class of membrane-bound E3 ligases.
"cells expressing either MIR1 or MIR2 display ubiquitination of their endogenous MHC-I chains"
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The N-terminal cytosolic Zn finger, then classified as PHD/LAP, is essential for MHC class I downregulation, as is a second element in the C-terminal cytosolic domain.
"The NH2-terminal region of both proteins harbors a Zn finger of the PHD/LAP subfamily"
Ubiquitylation of MHC class I by the K3 viral protein signals internalization and TSG101-dependent degradation.
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K3 associates with MHC class I and ubiquitylates it after ER export; the mark is the internalisation signal at the plasma membrane and routes class I into the TSG101-dependent late endosomal pathway for degradation.
"promotes ubiquitylation of class I after export from the endoplasmic reticulum."
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The ubiquitin mark on class I is the signal for its internalisation at the plasma membrane; UniProt additionally records from this paper that mutating the conserved Trp41 of the RING-CH domain abolishes ubiquitination activity and class I degradation.
"signal for class I internalization at the plasma membrane"
Solution structure of the Kaposi's sarcoma-associated herpesvirus K3 N-terminal domain reveals a Novel E2-binding C4HC3-type RING domain.
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The K3 N-terminal domain is a variant RING, not a plant homeodomain, and retains the canonical two-zinc cross-brace arrangement (PDB 1VYX).
"binding two zinc ions in a stable cross-brace motif"
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The domain docks the cellular E2 enzymes UbcH5A to UbcH5C and UbcH13 on the surface equivalent to that used by classical cellular RING domains, suggesting K3 may catalyse Lys63-linked ubiquitination.
"found to interact with the cellular ubiquitin-conjugating enzymes UbcH5A to -C"
Ubiquitination on nonlysine residues by a viral E3 ubiquitin ligase.
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MIR1 (K3), but not MIR2 (K5), downregulates MHC class I molecules whose intracytoplasmic domain lacks lysines; this requires a cysteine in the tail and the ubiquitin linkage is beta-mercaptoethanol-sensitive, establishing non-lysine ubiquitination.
"ubiquitination required a cysteine residue in the intracytoplasmic tail of MHC I"
Herpesviral protein networks and their interaction with the human proteome.
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Comprehensive intraviral yeast two-hybrid screens of KSHV and VZV reported 123 and 173 interactions respectively; the two K3 interactions annotated from this work (with P88941 and P88951) are network-level hits with no functional follow-up.
"The comprehensive yeast two-hybrid analysis of intraviral protein interactions"
Downregulation of gamma interferon receptor 1 by Kaposi's sarcoma-associated herpesvirus K3 and K5.
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K3 and K5 both specifically target IFNGR1 for ubiquitination, endocytosis and degradation, lowering its surface expression; K5 does so more strongly than K3.
"K3 and K5 both specifically target IFN-γR1 and induce its ubiquitination, endocytosis, and degradation, resulting in downregulation of IFN-γR1 surface expression"
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The functional consequence is proximal: IFN-gamma-induced STAT1 Tyr701 phosphorylation is markedly reduced in K3-expressing BJAB cells while total STAT1 is unchanged.
"tyrosine-phosphorylated STAT1 (pY701-STAT1) was markedly reduced in BJAB-K3 and BJAB-K5 cells"
Kaposi's sarcoma-associated herpesvirus K3 and K5 proteins down regulate both DC-SIGN and DC-SIGNR.
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Exogenously expressed wild-type K3 lowers surface DC-SIGN and DC-SIGNR by about 60% in 293 stable lines, and mutating the RING-CH zinc ligands abolishes the effect (Fig. 2).
"both K3 and K5 caused a reduction in surface expression of both DC-SIGN and DC-SIGNR by approximately 60%"
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The same experiment records K3's effect on endogenous MHC class I: surface levels fall about 80-90% with wild-type K3 and only 20-30% with the RING-CH mutant.
"In the presence of wild-type K3 or K5, surface levels of MHC I were reduced about 80–90%."
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DC-SIGN is ubiquitylated only in the presence of RING-CH-intact K3, and its degradation runs primarily through a ubiquitylation-dependent proteasomal route with a lysosomal fraction (Fig. 7A, 7B).
"We confirmed that DC-SIGN displayed ubiquitylation only in the presence of K3 or K5 with an intact RING-CH domain and not in the presence K3 mZn or K5 mZn."
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Dynasore washout shows K3 accelerates endocytosis of DC-SIGN but not of DC-SIGNR, and the tyrosine-based endocytosis motif is required in addition to the RING-CH domain (Fig. 4).
"K3 expression also caused increased endocytosis of DC-SIGN relative to vector expressing cells"
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In the context of infection the activity is K5's: viruses carrying a functional K5 gene downregulate DC-SIGN, DC-SIGNR and MHC class I, whereas the K5-deleted and doubly deleted viruses only weakly do so.
"Each of the viruses encoding a functional K5 gene (wt and ΔK3) showed significant down regulation of DC-SIGN, DC-SIGNR and MHC I from the cell surface."
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The entry-enhancing co-receptor role belongs to the host lectins and was measured with virus deleted for both K3 and K5 (Fig. 1), so it is not an activity of K3.
"It has been documented that DC-SIGN can act a co-receptor for KSHV, not sufficient for entry by itself, but capable of enhancing viral infectivity"
Kaposi's sarcoma-associated herpesvirus K3 and K5 ubiquitin E3 ligases have stage-specific immune evasion roles during lytic replication.
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Using the RGB-BAC16 reporter virus and K3/K5 locus-swap recombinants, K3 and K5 are shown to act sequentially rather than redundantly: K5 immediately on entry into the lytic phase and K3 during later stages of lytic replication.
"while K5 acts immediately upon entry into the lytic phase, K3-mediated downregulation of MHC-I was evident during later stages of lytic replication"