The Sec6/8 complex, also called the exocyst complex, forms an octameric protein (Sec3, Sec5, Sec6, Sec8, Sec10, Sec15, Exo70 and Exo84) involved in the tethering of secretory vesicles to specific regions on the plasma membrane. The regulation of Sec6/8 complex differs between mammals and yeast. Mamalian Exo84 and Sec5 are effector targets for active Ral GTPases which are not present in yeast. Ral GTPases are members of the Ras superfamily, and as such cycle between an active GTP-bound state and an inactive GDP-bound state. The Exo84 Ral-binding domain adopts a PH domain fold. Mammalian Exo84 and Sec5 competitively bind to active RalA. 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.",