TOMATO RAB1A HOMOLOGS AS MOLECULAR TOOLS FOR STUDYING RAB GERANYLGERANYL TRANSFERASE IN PLANT-CELLS

Rab proteins attach to membranes along the secretory pathway where the y contribute to distinct steps in vesicle-mediated transport. To bind membranes, Rab proteins in fungal and animal cells must be isoprenylat ed by the enzyme Rab geranylgeranyl transferase (Rab GGTase). We have isolated three tomato (Lycopersicon esculentum, M.) cDNAs (LeRab1A, B, and C) encoding Rab-like proteins and show here that all three are su bstrates for a Rab GGTase-like activity in plant cells. The plant enzy me is similar to mammalian Rab GGTase in that the plant activity (a) i s enhanced by detergent and (b) is inhibited by mutant Rab lacking a p renylation consensus sequence. LeRab1B contains a rare prenylation tar get motif and was the best substrate for the plant, but not the yeast, Rab GGTase. LeRab1A, B, and C are functional homologs of the Saccharo myces cerevisiae Rab protein encoded by YPT1 and are differentially ex pressed in tomato. LeRab1A mRNA, but not that of LeRab1B or C, is indu ced by ethylene in tomato seedlings and is also upregulated in ripenin g fruit. The increase in LeRab1A mRNA expression in ripe fruit may be linked to increased synthesis and export of enzymes like polygalacturo nase, pectin esterase, and other enzymes important in fruit softening.