Four phosphate adaptor protein 1 and 2 Pleckstrin homology (PH) domain
Human FAPP1 (also called PLEKHA3/Pleckstrin homology domain-containing, family A member 3) regulates secretory transport from the trans-Golgi network to the plasma membrane. It is recruited through binding of PH domain to phosphatidylinositol 4-phosphate (PtdIns(4)P) and a small GTPase ADP-ribosylation factor 1 (ARF1). These two binding sites have little overlap the FAPP1 PH domain to associate with both ligands simultaneously and independently. FAPP1 has a N-terminal PH domain followed by a short proline-rich region. FAPP1 is a member of the oxysterol binding protein (OSBP) family which includes OSBP, OSBP-related proteins (ORP), and Goodpasture antigen binding protein (GPBP). They have a wide range of purported functions including sterol transport, cell cycle control, pollen development and vessicle transport from Golgi recognize both PI lipids and ARF proteins. FAPP2 (also called PLEKHA8/Pleckstrin homology domain-containing, family A member 8), a member of the Glycolipid lipid transfer protein(GLTP) family has an N-terminal PH domain that targets the TGN and C-terminal GLTP domain. FAPP2 functions to traffic glucosylceramide (GlcCer) which is made in the Golgi. It's interaction with vesicle-associated membrane protein-associated protein (VAP) could be a means of regulation. Some FAPP2s share the FFAT-like motifs found in GLTP. 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.
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