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Obscurin pleckstrin homology (PH) domain Obscurin (also called Obscurin-RhoGEF; Obscurin-myosin light chain kinase/Obscurin-MLCK) is a giant muscle protein that is concentrated at the peripheries of Z-disks and M-lines. It binds small ankyrin I, a component of the sarcoplasmic reticulum (SR) membrane. It is associated with the contractile apparatus through binding with titin and sarcomeric myosin. It plays important roles in the organization and assembly of the myofibril and the SR. Obscurin has been observed as alternatively-spliced isoforms. The major isoform in sleletal muscle, approximately 800 kDa in size, is composed of many adhesion modules and signaling domains. It harbors 49 Ig and 2 FNIII repeats at the N-terminues, a complex middle region with additional Ig domains, an IQ motif, and a conserved SH3 domain near RhoGEF and PH domains, and a non-modular C-terminus with phosphorylation motifs. The obscurin gene also encodes two kinase domains, which are not part of the 800 kDa form of the protein, but is part of smaller spliced products that present in heart muscle. 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.",