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UBA domain of eukaryotic protein linking integrin-associated protein (IAP, also known as CD47) with cytoskeleton (PLIC) proteins The PLIC proteins (or ubiquilins) family contains human homologs of the yeast ubiquitin-like Dsk2 protein, PLIC-1 (also called ubiquilin-1), PLIC-2 (also called ubiquilin-2 or Chap1), PLIC-3 (also called ubiquilin-3) and PLIC-4 (also called ubiquilin-4, Ataxin-1 interacting ubiquitin-like protein, A1Up, Connexin43-interacting protein of 75 kDa, or CIP75), and mouse PLIC proteins. They are ubiquitin-binding adaptor proteins involved in all protein degradation pathways through delivering ubiquitinated substrates to proteasomes. They also promote autophagy-dependent cell survival during nutrient starvation. PLIC-1 regulates the function of the thrombospondin receptor CD47 and G protein signaling. It plays a role in TLR4-mediated signaling through interacting with the Toll/interleukin-1 receptor (TIR) domain of TLR4. It also inhibits the TLR3-Trif antiviral pathway by reducing the abundance of Trif. Moreover, PLIC-1 binds to gamma-aminobutyric acid receptors (GABAARs) and modulates the ubiquitin-dependent, proteasomal degradation of GABAARs. Furthermore, PLIC-1 acts as a molecular chaperone regulating amyloid precursor protein (APP) biosynthesis, trafficking, and degradation by stimulating K63-linked polyubiquitination of lysine 688 in the APP intracellular domain. In addition, PLIC-1 is involved in the protein aggregation-stress pathway via associating with the ubiquitin-interacting motif (UIM) proteins ataxin 3, HSJ1a, and epidermal growth factor substrate 15 (EPS15). PLIC-2 is a protein that binds the ATPase domain of the HSP70-like Stch protein. It functions as a negative regulator of G protein-coupled receptor (GPCR) endocytosis. It also involved in amyotrophic lateral sclerosis (ALS)-related dementia. PLIC-3 is encoded by UBQLN3, a testis-specific gene. It shows high sequence similarity with the Xenopus protein XDRP1, a nuclear phosphoprotein that binds to the N-terminus of cyclin A and inhibits Ca2+-induced degradation of cyclin A, but not cyclin B. PLIC-4 is a ubiquitin-like nuclear protein that interacts with ataxin-1 and further links ataxin-1 with the chaperone and ubiquitin-proteasome pathways. It also binds to the non-ubiquitinated gap junction protein connexin43 (Cx43) and regulates the turnover of Cx43 through the proteasomal pathway. PLIC proteins contain an N-terminal ubiquitin-like (UBL) domain that is responsible for the binding of ubiquitin-interacting motifs (UIMs) expressed by proteasomes and endocytic adaptors, and C-terminal ubiquitin-associated (UBA) domain that interacts with ubiquitin chains present on proteins destined for proteasomal degradation. In addition, mammalian PLIC2 proteins have an extra collagen-like motif region which is absent in other PLIC proteins and the yeast Dsk2 protein. |
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