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unc89 pleckstrin homology (PH) domain unc89 is a myofibrillar protein. unc89-B the largest isoform is composed of 53 immunoglobulin (Ig) domains, 2 Fn3 domains, a triplet of SH3, DH and PH domains at its N-terminus, and 2 protein kinase domains (PK1 and PK2) at its C-terminus. unc-89 mutants display disorganization of muscle A-bands, and usually lack M-lines. The COOH-terminal region of obscurin, the human homolog of unc89, interacts via two specific Ig-like domains with the NH(2)-terminal Z-disk region of titin, a protein that connects the Z line to the M line in the sarcomere and contributes to the contraction of striated muscle. obscurin is also thought to be involved in Ca2+/calmodulin via its IQ domains, as well as G protein-coupled signal transduction in the sarcomere via its RhoGEF/DH domain. The DH-PH region of OBSCN and unc89, the C. elegans homolog, has exchange activity for RhoA and Rho-1 respectively, but not for the small GTPases homologous to Cdc42 or Rac. PH domains have diverse functions, but in general are involved in targeting proteins to the appropriate cellular location or in the interaction with a binding partner. They share little sequence conservation, but all have a common fold, which is electrostatically polarized. Less than 10% of PH domains bind phosphoinositide phosphates (PIPs) with high affinity and specificity. PH domains are distinguished from other PIP-binding domains by their specific high-affinity binding to PIPs with two vicinal phosphate groups: PtdIns(3,4)P2, PtdIns(4,5)P2 or PtdIns(3,4,5)P3 which results in targeting some PH domain proteins to the plasma membrane. A few display strong specificity in lipid binding. Any specificity is usually determined by loop regions or insertions in the N-terminus of the domain, which are not conserved across all PH domains. PH domains are found in cellular signaling proteins such as serine/threonine kinase, tyrosine kinases, regulators of G-proteins, endocytotic GTPases, adaptors, as well as cytoskeletal associated molecules and in lipid associated enzymes. |
Jiyao W et al.(2023). "The conserved domain database in 2023.",