Ceramide-binding START domain of mammalian STARD11 and related domains
This subfamily includes the steroidogenic acute regulatory protein (StAR)-related lipid transfer (START) domains of mammalian STARD11 and related domains. The START domain family belongs to the SRPBCC (START/RHO_alpha_C/PITP/Bet_v1/CoxG/CalC) domain superfamily of proteins that bind hydrophobic ligands. SRPBCC domains have a deep hydrophobic ligand-binding pocket. STARD11 can mediate transfer of the natural ceramide isomers, dihydroceramide and phytoceramide, as well as ceramides having C14, C16, C18, and C20 chains. They can also transfer diacylglycerol, but with a lower efficiency. STARD11 is synthesized from two major transcripts: a larger one encoding Goodpasture antigen-binding protein (GPBP)/ceramide transporter long form (CERTL); and a smaller one encoding GPBPdelta26/CERT, which is deleted for 26 amino acids. Both splicing variants mediate ceramide transfer from the ER to the Golgi, in a non-vesicular manner. It is likely that these two carry out different functions in specific sub-cellular locations. These proteins have roles in brain homeostasis and disease processes. GPBP/CERTL exists in multiple isoforms originating from alternative translation initiation sites and post-translational modifications. Goodpasture syndrome is a human disorder caused by antibodies directed against the a3-chain of collagen type IV. GPBP/CERTL binds and phosphorylates this antigen. The human gene encoding STARD11 is referred to as COL4A3BP referring to its collagen binding function. It is unknown whether the ceramide-transfer function of GPBP/CERTL is related to this collagen interaction. The expression of GPBP/CERTL is elevated in these and other spontaneous autoimmune disorders including cutaneous lupus erythematosus, pemphigoid, and lichen planus. GPBL/CERTL contains an N-terminal pleckstrin homology domain (PH), which targets the protein to the Golgi, a middle region containing two serine-rich domains (SR1, SR2), a FFAT (two phenylalanine amino acids in an acidic tract) motif which is involved in endoplasmic reticulum targeting, and this C-terminal SMART domain. The shorter splicing variant, CERT, lacks the SR2 domain.