VAP-A: Proteins and Enzymes

Vesicle-associated membrane protein (VAMP)-associated protein A (VAP-A; also VAMP-A and VAP-33) is a 33 kDa, ubiquitously expressed, type IV transmembrane protein belonging to the VAP family of proteins. It is found in plasma and ER membranes as well as in intracellular vesicles as a homodimer and a heterodimer with VAP-B. Human VAP-A is synthesized as a 249 amino acid (aa) precursor that contains a 227 aa cytoplasmic domain and a 21 aa transmembrane region. The cytoplasmic domain contains a mobile sperm protein (MSP) domain (aa 14-131) and a coiled-coil region (aa 169-205). Human VAP-A is 97% aa identical to mouse and rat VAP-A. VAP-A and VAP-B recruit FFAT (two phenylalanines in an acidic tract)-motif-containing proteins to the cytosolic surface of ER membranes through a conserved region within their MSP domain, and they have been implicated in regulation of membrane transport, phospholipid biosynthesis, and the unfolded protein response. Their role in maintaining the identities of intracellular organelles has not been demonstrated, but their ability to interact with lipid-transfer/binding proteins (LT/BPs) may affect the lipid composition of certain cellular membranes. One study shows that VAPs play a critical role in maintaining the structural and functional properties of the Golgi complex. Researchers found that knockdown of VAP reduces the levels of phosphatidylinositol-4-phosphate (PI4P), diacylglycerol (DAG), and sphingomyelin (SM) in Golgi membranes and exports pleiotropic effects in Golgi-mediated transport. The effects of VAPs are mediated by their interacting FFAT-motif-containing proteins Nir2, OSBP, and CERT. VAPs provide a scaffold for these LT/BPs at the ER-Golgi membrane contact sites, thereby affecting the lipid composition of the Golgi membranes and consequently their structural and functional identities. Most recently, researchers found that VAP-A associates and co-localizes with protrudin, a protein that promotes neurite formation, and found that it was an important regulator both of the subcellular localization of protrudin and of its ability to stimulate neurite outgrowth.