NOVEL PI(4)P 5-KINASE HOMOLOG, FAB1P, ESSENTIAL FOR NORMAL VACUOLE FUNCTION AND MORPHOLOGY IN YEAST

The FAB1 gene of budding yeast is predicted to encode a protein of 257 kDa that exhibits significant sequence homology to a human type II PI (4)P 5-kinase (PIP5K-II). The recently cloned human PIP5K-II specifica lly converts PI(4)P to PI(4, 5)P-2 (Boronenkov and Anderson, 1995). Th e region of highest similarity between Fab1p and PIP5K-II includes a p redicted nucleotide binding motif, which is likely to correspond to th e catalytic domain of the protein. Interestingly, neither PIP5K-II nor Fab1p exhibit significant homology with cloned PI 3-kinases or PI 4-k inases. fab1 mutations result in the formation of aploid and binucleat e cells (hence the name FAB). In addition, loss of Fab1p function caus es defects in vacuole function and morphology, cell surface integrity, and cell growth. Experiments with a temperature conditional fab1 muta nt revealed that their vacuoles rapidly (within 30 min) enlarge to mor e than double the size upon shifting cells to the nonpermissive temper ature. Additional experiments with the fab1 ts mutant together with re sults obtained with fab1 vps (vacuolar protein sorting defective) doub le mutants indicate that the nuclear division and cell surface integri ty defects observed in fab1 mutants are secondary to the vacuole morph ology defects. Based on these data, we propose that Fab1p is a PI(4)P 5-kinase and that the product of the Fab1p reaction, PIP,, functions a s an important regulator of vacuole homeostasis perhaps by controlling membrane flux to and/or from the vacuole. Furthermore, a comparison o f the phenotypes of fab1 mutants and other yeast mutants affecting PI metabolism suggests that phosphoinositides may serve as general regula tors of several different membrane trafficking pathways.