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Published 19 December 2005. doi:10.1083/jcb.200504104
The Rockefeller University Press, 0021-9525 $8.00
JCB, Volume 171, Number 6, 967-979
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Article

 Yeast phosphatidylinositol 4-kinase, Pik1, has essential roles at the Golgi and in the nucleus

Thomas Strahl1, Hiroko Hama2, Daryll B. DeWald2, and Jeremy Thorner1

1 Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, CA 94720
2 Department of Biology, Utah State University, Logan, UT 84322

Correspondence to Jeremy Thorner: jthorner{at}berkeley.edu

Phosphatidylinositol 4-kinase, Pik1, is essential for viability. GFP-Pik1 localized to cytoplasmic puncta and the nucleus. The puncta colocalized with Sec7-DsRed, a marker of trans-Golgi cisternae. Kap95 (importin-ß) was necessary for nuclear entry, but not Kap60 (importin-{alpha}), and exportin Msn5 was required for nuclear exit. Frq1 (frequenin orthologue) also is essential for viability and binds near the NH2 terminus of Pik1. Frq1-GFP localized to Golgi puncta, and Pik1 lacking its Frq1-binding site (or Pik1 overexpressed in frq1{Delta} cells) did not decorate the Golgi, but nuclear localization was unperturbed. Pik1({Delta}10-192), which lacks its nuclear export sequence, displayed prominent nuclear accumulation and did not rescue inviability of pik1{Delta} cells. A Pik1-CCAAX chimera was excluded from the nucleus and also did not rescue inviability of pik1{Delta} cells. However, coexpression of Pik1({Delta}10-192) and Pik1-CCAAX in pik1{Delta} cells restored viability. Catalytically inactive derivatives of these compartment-restricted Pik1 constructs indicated that PtdIns4P must be generated both in the nucleus and at the Golgi for normal cell function.

H. Hama's present address is Dept. of Biochemistry and Molecular Biology, Medical University of South Carolina, Charleston, SC 29425.

Abbreviations used in this paper: DsRed, Discosoma spp. red fluorescent protein; GEF, guanine nucleotide exchange factor; IP3, inositol-1,4,5-P3; IPxs, other inositol-phosphates; PH, pleckstrin homology; PI4K, PtdIns 4-kinase; PIP, phosphoinositide; PM, plasma membrane; PtdIns, phosphatidylinositol.


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