FAB1 encodes a phosphatidylinositol(3)-phosphate 5-kinase responsible for synthesis of phosphatidylinositol 3,5-bisphosphate (PtdIns(3,5)P2) from phosphatidylinositol 3-phosphate (PtdIns(3)P; 3). Fab1p localizes to the vacuolar membrane, and is the sole kinase that synthesizes PtdIns(3,5)P2 (1, 4, 5). PtdIns(3,5)P2 is required for proper trafficking of endocytic cargo through the late endosome/multivesicular body to the vacuole lumen (6, 7, 8) and is involved in the cellular response to osmotic changes in the environment, accumulating to levels 20-fold higher than basal upon hyperosmotic stress (9).

Loss of Fab1p function leads to a variety of phenotypes, including formation of aploid and binucleate cells (hence, the name FAB1) due to improper orientation of the mitotic spindle (1). Genetic studies indicate that the large vacuole size exhibited by these mutants causes the abnormal chromosome distribution (1). Deletion mutants exhibit growth defects at 23 degrees, inviability at 37 degrees, and enlarged vacuoles at both temperatures. Further, these mutants do not properly acidify their vacuoles and lack detectable levels of PtdIns(3,5)P2 (1, 5). Overexpression of Fab1p does not increase levels of PtdIns(3,5)P2, suggesting tight regulation of this enzyme (5). Fab1p activity is activated by Vac7p, a vacuolar membrane protein that appears to have homologs only in fungi, and independently by the Vac14p-Fig4p complex, which is required for turnover of PtdIns(3,5)P2 (Fig4p is a polyphosphoinositide phosphatase; 10, 11).

S. cerevisiae contains two genes that encode phosphatidylinositol phosphate kinases (PIPkins): FAB1, and MSS4, which enodes a phosphatidylinositol-4-phosphate 5-kinase (12, 3). Fab1p orthologs have been identified in S. pombe (13), mouse (14), C. elegans (15), and humans (16), among others. Mutations in the human ortholog PIP5K3 are associated with Francois-Neetens fleck corneal dystrophy (CFD), a rare corneal disorder characterized by numerous small white flecks scattered throughout the layers of the stroma (17). Collectively, the Fab1p orthologs have 4 domains in common: the FYVE domain, which has been shown to bind the substrate Ptdins3P (18), the TCP-1/chaperonin-like domain, which appears to be required for catalytic activity, a cysteine-rich domain, the role of which is uncertain, and a PIPkin domain, which is responsible for catalytic activity (reviewed in 19).

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 20 and 21). PtdInsPs are also precursors of the water-soluble inositol phosphates (IPs), an important class of intracellular signaling molecules (reviewed in 22, 23 and 24).

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 20). These stereoisomers have been shown to be restricted to certain membranes (reviewed in 20). 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 20). 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 20). 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 20).

Phosphorylation and dephosphorylation of the inositol headgroups of PtdInsPs at specific membrane locations signals the recruitment of certain proteins essential for vesicular transport (21, and reviewed in 20). 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 20).

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 20). 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: 2007-03-02