SUMMARY PARAGRAPH for PIK1
Phosphatidylinositol 4-kinases (ATP:phosphatidylinositol-4-phosphotransferase, EC 220.127.116.11) are evolutionarily conserved enzymes that catalyze the formation of phosphatidylinositol 4-phosphate and ADP from phosphatidylinositol (PtdIns or PI) and ATP, the first step in the synthesis of phosphatidylinositol phosphates (5, 6). Two types of PtdIns 4-kinases (types II and III) have been identified based on their biochemical properties (5). Type III PtdIns 4-kinases all contain a common catalytic kinase domain, which is also found in type I PtdIns 3-kinases (7, 8). There are two type III PtdIns 4-kinases in S. cerevisiae, encoded by PIK1 and STT4 (1, 9). LSB6 encodes the single type II PtdIns 4-kinase in yeast (5, 6).
Pik1p is a soluble 125-kDa enzyme (10), and Stt4p is a plasma membrane-associated 215-kDa enzyme (9, 11). Together, Pik1p and Stt4p account for the vast majority of PtdIns 4-kinase activity in wild-type yeast cells (12). The two different type III PtdIns 4-kinases synthesize discrete pools of PtdIns 4-phosphate with essential roles in cell physiology (12). Overproduction of one of these type III PtdIns 4-kinases cannot compensate for a gene disruption in the other (12). Stt4p is homologous to mammalian PtdIns 4-kinase alpha, and Pik1p to mammalian PtdIns 4-kinase beta (7).
Pik1p localizes to the nucleus and to the Golgi, where it regulates secretion (3, 12, 13, 14, 15, 16). Pik1p is also required for proper Golgi and vacuole membrane dynamics, and endocytosis (13, 12, 6). Pik1p is activated by frequenin (Frq1p), which binds Pik1p at its amino terminus, and aids in targeting Pik1p to the Golgi (17, 18, 15, 19). Pik1p also binds the 14-3-3 phosphorylation-dependent protein chaperones Bmh1p and Bmh2p (20). This interaction occurs primarily in the cytoplasm, and controls the nucleocytoplasmic shuttling and availability of Pik1p for Golgi recruitment (20). Nutrient deprivation results in a relocation of Pik1p from the Golgi to the nucleus, which is rapidly reversed upon restoration of nutrient supply (20).
pik1 null mutants are inviable, and conditional pik1 mutants exhibit defects in cytokinesis, endocytosis, protein secretion, and vacuolar dynamics (3, 1, 12, 6). Temperature-sensitive pik1 mutants also display aberrant Golgi morphology (12), and can fail to undergo meiosis and spore formation (21). Inactivation of Pik1p results in severe translation initiation defects and a marked increased in the phosphorylation of translation initiation factor eIF2alpha (22). Overexpression of PIK1 produces defects in membrane trafficking (23).
About Phosphatidylinositol Phosphate Biosynthesis
The phosphorylated products of phosphatidylinositol (PtdIns, PI), collectively referred to as phosphoinositides or phosphatidylinositol phosphates (PtdInsPs, PIPs), are membrane-bound lipids that function as structural components of membranes, as well as regulators of many cellular processes in eukaryotes, including vesicle-mediated membrane trafficking, cell wall integrity, and actin cytoskeleton organization (reviewed in 24 and 25). PtdInsPs are also precursors of the water-soluble inositol phosphates (IPs), an important class of intracellular signaling molecules (reviewed in 26, 27 and 28).
The inositol ring of the membrane phospholipids and the water-soluble IPs are readily phosphorylated and dephosphorylated at a number of positions making them well suited as key regulators. PtdIns can be phosphorylated at one or a combination of positions (3', 4', or 5') on the inositol headgroup, generating a set of unique stereoisomers that have specific biological functions (reviewed in 24). These stereoisomers have been shown to be restricted to certain membranes (reviewed in 24). Phosphatidylinositol 4-phosphate (PtdIns4P) is the major PtdInsP species of the Golgi apparatus, where it plays a role in the vesicular trafficking of secretory proteins from the Golgi to the plasma membrane (reviewed in 24). Phosphatidylinositol 4,5-bisphosphate (PtdIns[4,5]P2) is the major species found at the plasma membrane and is involved in the regulation of actin cytoskeleton organization, as well as cell wall integrity, and heat shock response pathways (reviewed in 24). Phosphatidylinositol 3-phosphate (PtdIns3P) is found predominantly at endosomal membranes and in multivesicular bodies (MVB), where it plays a role in endosomal and vacuolar membrane trafficking. Phosphatidylinositol 3,5-bisphosphate (PtdIns[3,5]P2) is found on vacuolar membranes where it plays an important role in the MVB sorting pathway (reviewed in 24).
Phosphorylation and dephosphorylation of the inositol headgroups of PtdInsPs at specific membrane locations signals the recruitment of certain proteins essential for vesicular transport (25, and reviewed in 24). PtdInsPs recruit proteins that contain PtdInsP-specific binding domains, such as the well-studied pleckstrin homology (PH) domain that recognizes the phosphorylation pattern of specific PtdInsP inositol headgroups (reviewed in 24).
A number of kinases and phosphatases are involved in the generation and interconversions of PtdInsPs, the majority of which have been well conserved during evolution (reviewed in 24). The PtdInsP kinases, in contrast to the lipid phosphatases, have a higher degree of specificity. While each kinase appears to phosphorylate only one substrate, many of the lipid phosphatases can dephosphorylate a number of substrates.
Last updated: 2008-05-08