Posttranslational prenylation of proteins is a widespread phenomenon and the majority of prenylated proteins are geranylgeranylated members of the Rab GTPase family. Geranylgeranylation is catalyzed by Rab geranylgeranyltransferase (RabGGTase) and is critical for the ability of Rab protein to mediate vesicular docking and fusion of various intracellular vesicles. RabGGTase consists of a catalytic alpha/beta heterodimer and an accessory protein termed Rab escort protein (REP-1) that delivers the newly prenylated Rab proteins to their target membrane. Mutations in the REP-1 gene in humans lead to an X-chromosome-linked defect known as choroideremia--a debilitating disease that inevitably culminates in complete blindness. Here we report in vitro assembly and purification of the stoichiometric ternary complex of RabGGTase with REP-1 stabilized by a hydrolysis-resistant phosphoisoprenoid analog--farnesyl phosphonyl(methyl)phoshonate. The complex formed crystals of extended plate morphology under low ionic-strength conditions. X-ray diffraction data were collected to 2.8 A resolution at the ESRF. The crystals belong to the monoclinic space group P2(1), with unit-cell parameters a = 68.7, b = 197.7, c = 86.1 A, beta = 113.4 degrees. Preliminary structural analysis revealed the presence of one molecule in the asymmetric unit.