GGTase-II (RabGGTase)

GGTase-II (Geranylgeranyltransferase-II) catalyzes the transfer of geranylgeranyl moiety onto two C-terminal cysteines of Rab proteins. Composed of an α and β heterodimer (50 and 38 kDa, respectively) and requires Rab escort protein for its catalytic activity. GGTase-II was shown to exhibit higher affinity towards geranylgeranyl pyrophosphate (K d= 8 nM) than farnesyl pyrophosphate (K d= 60 nM). Like FTase and GGTase-I, RabGGT functions as a heterodimer. The α subunit has 27% identity to that of CaaX prenylases but contains additional domains, while the β subunit shows 29% identity to that of FTase. The protein substrates of RabGGT have heterogeneous C termini that usually contain two cysteine residues (CXC), both of which are modified by geranylgeranyl groups. Unlike the CaaX prenylases, RabGGT requires specific accessory proteins known as REPs to guide the interaction with its targets. Some farnesyltransferase inhibitors (FTIs) were identified to inhibit RabGGT activity and induce p53 independent apoptosis in C. elegans.