MSS4 encodes a phosphatidylinositol (PtdIns) 4-phosphate 5-kinase that catalyzes the phosphorylation of PtdIns 4-phosphate, the final step in the synthesis of phosphatidylinositol 4,5-bisphosphate (PtdIns[4,5]P2) (4, 2). Mss4p acts with the PtdIns 4-kinase Stt4p at the plasma membrane to generate PtdIns[4,5]P2, which is required for activation of the Rho1/Pkc1-mediated MAP kinase cascade important for normal organization of the actin cytoskeleton (5, 6). Mss4p activity and synthesis of PtdIns[4,5]P2 is also essential for sporulation, endocytosis, and membrane trafficking (7, 8). Mss4p contains a functional nuclear localization signal (NLS), and undergoes nucleocytoplasmic shuttling, thereby regulating PtdIns[4,5]P2 synthesis at the plasma membrane (6). Nucleocytoplasmic shuttling of Mss4p depends on Kap123p and Bcp1p, whereas its recruitment to the plasma membrane involves phosphorylation by the casein kinase I isoforms Yck1p and Yck2p (9). An essential role for Mss4p in the nucleus has not been demonstrated (6). Treatment of cells with ISP-1/myriocin causes defects in both the activity and subcellular localization of Mss4p (10). Mss4p is also mislocalized in cells lacking functional Csg2p, an ER-membrane protein required for mannosylation of inositolphosphorylceramide (10).

MSS4 was originally identified as a multicopy suppressor of a temperature-sensitive mutation in the STT4 PtdIns 4-kinase (1). Inactivation of either Stt4p or Mss4p results in mislocalization of the Rho-GTPase guanine nucleotide exchange factor Rom2p (5), and a shift from polarized to isotropic cell growth, such that cells severely increase in size and adopt a round shape (2). Mutations in either STT4 or MSS4 display synthetic defects with a number of mutations in genes encoding known components of the PKC1 pathway (5). mss4 null mutants are inviable, exhibiting aberrant filamentous actin localization and subsequent cell death (1, 3).

Temperature-sensitive mss4 mutant cells have only a fraction of the normal amount of PtdIns[4,5]P2, are unable to form actin cables or properly localize their actin cytoskeleton during polarized cell growth, and exhibit defective spore formation (3, 2, 7). Cells lacking functional Mss4p also partially mislocalize the PtdIns[4,5]P2-binding protein Slm1p to the cytoplasm (11, 12). Overexpression of MSS4 confers resistance to wortmannin, suppresses growth and actin defects associated with mutations in TOR2 and CMD1, and generates elevated levels of PtdIns[4,5]P2 (13, 14, 3, 15). MSS4 overexpression also alleviates the growth and secretory defects of a number of temperature-sensitive late secretion mutants, including various components of the exocyst, such as SEC8, SEC10, and SEC15 (16).

Phosphatidylinositol 4-phosphate 5-kinase activity is conserved from yeast to human (2). Homologs of Mss4p have been identified in other yeasts, including Schizosaccharomyces pombe (17) and Candida albicans (18).

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 9 and 19). PtdInsPs are also precursors of the water-soluble inositol phosphates (IPs), an important class of intracellular signaling molecules (reviewed in 20, 21 and 22).

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 9). These stereoisomers have been shown to be restricted to certain membranes (reviewed in 9). 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 9). 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 9). 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 9).

Phosphorylation and dephosphorylation of the inositol headgroups of PtdInsPs at specific membrane locations signals the recruitment of certain proteins essential for vesicular transport (19, and reviewed in 9). 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 9).

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 9). 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