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

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-beta) 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.