PRELIMINARY X-RAY STRUCTURE ANALYSIS OF RAB GERANYLGERANYL TRANSFERASE. Hong Zhang, Miguel C. Seabra* and Johann Deisenhofer Howard Hughes Medical Institute and *Department of Molecular Genetics, University of Texas Southwestern Medical Center, Dallas, Texas 75235

Rab proteins comprise a family of small GTPase that play a key role in the regulation of vesicular membrane traffic. The attatchment of a 20 carbon isoprenoid, geranylgeranyl, to the C-terminal cysteine motifs of Rabs is crucial for their membrane association and function. This post-translational modification is a complex reaction that requires a catalytic Rab geranylgeranyl transferase (Rate GGTase) and a Rab escort protein (REP).

The Rab GGTase is a heterodimer consists of two tightly associated [[alpha]] and [[beta]] subunits of 65 and 37kD, respectively. Both subunits of GGTase have been cloned and co-expressed in the baculovirus/Sf9 insect cell system. The whole protein has been purified to homogeneity. Diffraction quality crystals have been obtained from buffered PEG solution using the vapor diffusion method. The crystals diffract to about 2.8Å resolution and they are of P1 space group with unit cell dimensions a=57.96Å, b=77.47Å, c=123.54Å, [[alpha]]=74.89deg., [[beta]]=80.39deg. and [[gamma]]=67.98deg.. We will present the recent progress in our attemp to determine the three dimesional structure of Rab GGTase.