High impact information on PIK1

• The sequence of the PIK1 gene product bears similarities to that of PtdIns 3-kinases from mammals (p110) and yeast (Vps34p) [1].
• Frq1 binds to a conserved sequence motif in Pik1 outside Pik1's catalytic domain and stimulates its activity in vitro [2].
• Frq1 (frequenin orthologue) also is essential for viability and binds near the NH2 terminus of Pik1 [3].
• We suggest that PIK1 controls cytokinesis through the actin cytoskeleton [4].
• Here we present the cloning and characterization of a gene from Saccharomyces cerevisiae, PIK1, encoding the enzyme that catalyses the first committed step in the production of the second messenger inositol-1,4,5-trisphosphate [4].

Biological context of PIK1

• Correspondingly, we found that PtdIns(4)P levels were decreased significantly in sec14-3 mutants shifted to 37 degrees C and that sec14-3 cells could grow at an otherwise nonpermissive temperature (34 degrees C) when carrying a plasmid overexpressing PIK1, encoding one of two essential PtdIns 4-kinases [5].
• Overexpression of Pik1p, a PtdIns 4-kinase, suppressed the sec14-1 meiosis and spore formation defects; conversely, pik1-ts diploids failed to undergo meiosis and spore formation [6].
• Distinct roles for the yeast phosphatidylinositol 4-kinases, Stt4p and Pik1p, in secretion, cell growth, and organelle membrane dynamics [7].
• In contrast, Pik1p is essential for normal secretion, Golgi and vacuole membrane dynamics, and endocytosis [7].
• Although Pik1p and Stt4p are closely related members of the Type III class of PI 4-kinases, Lsb6p belongs to the distinct Type II class, based on its amino acid sequence, its sensitivity to inhibition by adenosine and its insensitivity to wortmannin [8].

Anatomical context of PIK1

• Based on these results, we propose that Stt4p and Pik1p act as the major, if not the only, PtdIns 4-kinases in yeast and produce distinct pools of PtdIns(4)P and PtdIns(4,5)P(2) that act on different intracellular membranes to recruit or activate as yet uncharacterized effector proteins [7].
• Pik1p is important for exocytic transport from the Golgi, whereas Stt4p plays a role in cell-wall integrity and cytoskeletal rearrangements [8].
• Studies with mutants of phosphatidylinositol 4-kinase, PIK1, suggested that the interaction with membranes is facilitated by phosphoinositides [9].

Associations of PIK1 with chemical compounds

• Both gene products phosphorylate PtdIns at the D-4 position of the inositol ring to generate PtdIns(4)P, which plays an essential role in yeast viability because deletion of either STT4 or PIK1 is lethal [7].
• Cells carrying PIK1 on a multicopy vector overexpress PI 4-kinase activity exclusively in a nuclear fraction, suggesting that PIK1 is part of a nuclear phosphoinositide cycle [4].
• In Saccharomyces cerevisiae, the phosphatidylinositol 4-kinase, Pik1p generates a distinct pool of PtdIns(4)P that is required for normal Golgi structure and secretory function [10].
• In contrast, phosphatidylinositol 4-phosphate generated by Pik1p localized in intracellular compartments [11].

Other interactions of PIK1

• In vitro, the PI 4-kinase activity of STT4, but not of PIK1, was potently inhibited by wortmannin [12].
• These findings indicate that, first, PtdIns(4)P limitation is a major contributing factor to the secretory defect in sec14 cells; second, Sec14p function is coupled to the action of Pik1p, and; third, PtdIns(4)P has an important role in the Golgi-to-plasma membrane stage of secretion [5].
• Thus, rather than a role for PtdOH in stimulating PtdIns(4,5)P2 formation, our findings indicate that during sporulation, Spo14p-mediated PtdOH production functions downstream of Sec14p-, Pik1p-, and Mss4p-dependent PtdIns(4,5)P2 synthesis [6].
• N-myristoylated Frq1 may also assist in Pik1 localization [2].
• This targeting is independent of Pik1p but dependent on the Golgi GTPase Arf1p [13].

Analytical, diagnostic and therapeutic context of PIK1

• The PIK1 gene encoding a PtdIns 4-kinase from the yeast Saccharomyces cerevisiae was isolated by polymerase chain reaction (PCR) with oligonucleotides based on the sequence of peptides derived from the purified enzyme [1].
• Site-directed mutagenesis indicates that the binding determinant in Pik1 is a hydrophobic alpha-helix and that frequenins bind to one side of this alpha-helix [14].
• In cell extracts, Pik1 and Frq1 exist mainly in a heterodimeric complex, as shown by size exclusion chromatography [15].

References