A protein that stimulates the GTPase activity of a small GTP-binding protein and switches it to the inactive (GDP-bound) state. The first GAP was purified as a 125-kDa protein from bovine brain and activated GTPase activity of ras-p21; it would now be termed ras-GAP. GAPs may be regulated by phospholipids and by phosphorylation on a tyrosine residue by growth factor receptors (PDGF-R, EGF-R). The neurofibromatosis type 1 gene (NF1) codes for a protein homologous to ras-GAP. The action of a GAP on a GTP-binding protein is reversed by GEFs, GTP-exchange factors that stimulate release of GDP and allow binding of GTP; this can be blocked by GDIs (guanosine nucleotide dissociation inhibitors) that prevent release of GDP.
Subjects: Medicine and Health.