Cryptosporidium-like Oxysterol binding protein related protein 2 Pleckstrin homology (PH) domain
There are 2 types of ORPs found in Cryptosporidium: CpORP1 and CpORP2. Cryptosporium differs from other apicomplexans like Plasmodium, Toxoplasma, and Eimeria which possess only a single long-type ORP consisting of an N-terminal PH domain followed by a C-terminal ligand binding (LB) domain. CpORP2 is like this, but CpORP1 differs and has a truncated N-terminus resulting in only having a LB domain present. The exact functions of these proteins are largely unknown though CpORP1 is thought to be involved in lipid transport across the parasitophorous vacuole membrane. Oxysterol binding proteins are a multigene family that is conserved in yeast, flies, worms, mammals and plants. In general OSBPs and ORPs have been found to be involved in the transport and metabolism of cholesterol and related lipids in eukaryotes. They all contain a C-terminal oxysterol binding domain, and most contain an N-terminal PH domain. OSBP PH domains bind to membrane phosphoinositides and thus likely play an important role in intracellular targeting. They are members of the oxysterol binding protein (OSBP) family which includes OSBP, OSBP-related proteins (ORP), Goodpasture antigen binding protein (GPBP), and Four phosphate adaptor protein 1 (FAPP1). 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. 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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