Ypt protein prenylation depends on the interplay among levels of Rab escort protein and geranylgeranyl diphosphate in yeast cells

Farnesyl diphosphate (FPP), an intermediate of the sterol biosynthetic path way, is used by farnesyl transferase to farnesylate, among others, the Ras proteins, and by geranylgeranyl diphosphate synthase to produce geranylgera nyl diphosphate (GGPP). GGPP is then transferred by geranylgeranyl transfer ase II (GGTase II) to Rab/Ypt members of the Ras superfamily known to be re quired at all stages of vesicle transport in both mammals and yeast. Format ion of a complex between a Rab/Ypt protein and an accessory protein named t he Rab escort protein (REP) is a prerequisite for GGTase II substrate recog nition. Little is known about the factors that regulate GGTase II activity in living cells but, based on available data, it seems possible that vesicl e transport in higher eukaryotes is regulated by the levels of prenylated R ab/Ypt proteins in the cells. Here we show that the levels of REP play an i mportant role in regulating GGTase II activity in yeast cells if sufficient substrates are present. Moreover, overexpression of REP causes, directly o r indirectly, an increased level of Ypt substrates available for prenylatio n, which in turn leads to the depletion of the GGPP pool in the cell. Overa ll our data suggest that the levels of REP and the availability of GGPP pla y a role in regulating Ypt protein prenylation. Copyright (C) 2001 John Wil ey & Sons, Ltd.