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J. Cell Biol.,
Volume 143, Number 1, October 5, 1998 65-79
* Division of Cellular and Molecular Medicine and Howard Hughes Medical Institute, University of California at San Diego,
School of Medicine, La Jolla, California 92093-0668; and § Department of Biochemistry, University of Iowa, Iowa City,
Iowa 52242
The Saccharomyces cerevisiae FAB1 gene
encodes a 257-kD protein that contains a cysteine-rich
RING-FYVE domain at its NH2-terminus and a kinase
domain at its COOH terminus. Based on its sequence, Fab1p was initially proposed to function as a phosphatidylinositol 4-phosphate (PtdIns(4)P) 5-kinase (). Additional sequence analysis of the
Fab1p kinase domain, reveals that Fab1p defines a subfamily of putative PtdInsP kinases that is distinct from
the kinases that synthesize PtdIns(4,5)P2. Consistent
with this, we find that unlike wild-type cells, fab1
,
fab1tsf, and fab1 kinase domain point mutants lack detectable levels of PtdIns(3,5)P2, a phosphoinositide recently identified both in yeast and mammalian cells.
PtdIns(4,5)P2 synthesis, on the other hand, is only moderately affected even in fab1
mutants. The presence of
PtdIns(3)P in fab1 mutants, combined with previous data, indicate that PtdIns(3,5)P2 synthesis is a two step
process, requiring the production of PtdIns(3)P by the
Vps34p PtdIns 3-kinase and the subsequent Fab1p-
dependent phosphorylation of PtdIns(3)P yielding
PtdIns(3,5)P2. Although Vps34p-mediated synthesis of PtdIns(3)P is required for the proper sorting of hydrolases from the Golgi to the vacuole, the production of
PtdIns(3,5)P2 by Fab1p does not directly affect Golgi to
vacuole trafficking, suggesting that PtdIns(3,5)P2 has a
distinct function. The major phenotypes resulting from
Fab1p kinase inactivation include temperature-sensitive growth, vacuolar acidification defects, and dramatic increases in vacuolar size. Based on our studies,
we hypothesize that whereas Vps34p is essential for anterograde trafficking of membrane and protein cargoes
to the vacuole, Fab1p may play an important compensatory role in the recycling/turnover of membranes deposited at the vacuole. Interestingly, deletion of VAC7
also results in an enlarged vacuole morphology and has
no detectable PtdIns(3,5)P2, suggesting that Vac7p
functions as an upstream regulator, perhaps in a complex with Fab1p. We propose that Fab1p and Vac7p are
components of a signal transduction pathway which
functions to regulate the efflux or turnover of vacuolar
membranes through the regulated production of PtdIns(3,5)P2.
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