Crystal structure of Rab geranylgeranyltransferase at 2.0 angstrom resolution

Background: Rab geranylgeranyltransferase (RabGGT) catalyzes the addition o f two geranylgeranyl groups to the C-terminal cysteine residues of Rab prot eins, which is crucial for membrane association and function of these prote ins in intracellular vesicular trafficking. Unlike protein farnesyltransfer ase (FT) and type I geranylgeranyltransferase, which both prenylate monomer ic small G proteins or short peptides, RabGGT can prenylate Rab only when R ab is in a complex with Rab escort protein (REP). Results: The crystal structure of rat RabGGT at 2.0 Angstrom resolution rev eals an assembly of four distinct structural modules. The beta subunit form s an alpha-alpha barrel that contains most of the residues in the active si te. The a subunit consists of a helical domain, an immunoglobulin (lg)-like domain, and a leucine-rich repeat (LRR) domain. The N-terminal region of t he alpha subunit binds to the active site in the beta subunit; residue His2 alpha directly coordinates a zinc ion. The prenyl-binding pocket of RabGGT is deeper than that in FT. Conclusions: LRR and lg domains are often involved in protein-protein inter actions; in RabGGT they might participate in the recognition and binding of REP. The binding of the N-terminal peptide of the alpha subunit to the act ive site suggests an autoinhibition mechanism that might contribute to the inability of RabGGT to recognize short peptides or Rab alone as its substra te. Replacement of residues Trp102 beta and Tyr154 beta in FT by Ser48 beta and Leu99 beta, respectively, in RabGGT largely determine the different li pid-binding specificities of the two enzymes.