MRCK (myotonic dystrophy-related Cdc42-binding kinase) pleckstrin homology (PH) domainMRCK is thought to be coincidence detector of signaling by Cdc42 and phosphoinositides. It has been shown to promote cytoskeletal reorganization, which affects many biological processes. There are 2 members of this family: MRCKalpha and MRCKbeta. MRCK consists of a serine/threonine kinase domain, a cysteine rich (C1) region, a PH domain and a p21 binding motif. The MRCK PH domain is responsible for its targeting to cell to cell junctions. 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.