SUMMARY PARAGRAPH for LSB6
Phosphatidylinositol 4-kinases (ATP:phosphatidylinositol-4-phosphotransferase, EC 18.104.22.168) 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 (2, 3). Two types of PtdIns 4-kinases (types II and III) have been identified based on their biochemical properties (2). Type III PtdIns 4-kinases all contain a common catalytic kinase domain, which is also found in type I PtdIns 3-kinases (4, 5). There are two type III PtdIns 4-kinases in S. cerevisiae, encoded by PIK1 and STT4 (6, 7). LSB6 encodes the single type II PtdIns 4-kinase in yeast (2, 3).
LSB6 is a non-essential gene whose PtdIns 4-kinase product is associated with plasma and vacuolar membranes (2). Lsb6p was originally identified as a protein that binds Las17p, an actin assembly factor that is the yeast homolog of the human Wiskott-Aldrich syndrome protein (1). Lsb6p regulates endosome motility by interacting with Las17p, a function for which its PI 4-kinase activity is dispensable (8). The Lsb6p amino terminus is highly conserved among fungi, and is required for binding Las17p (8). This interaction of Lsb6p with Las17p leads to activation of the Arp2/3 complex, actin filament assembly and endosome motility (8). lsb6 null mutants exhibit no defects in growth, phosphatidylinositol phosphate synthesis or vacuolar morphology (2, 3). However, they do exhibit significant impairment of endosome motility (8). Overexpression of LSB6 can partially suppress the lethal phenotype of the stt4 null mutant, but not that of the pik1 null mutant (2). Lsb6p is subject to palmitoylation, which is required for full kinase activity (3, 9).
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 10 and 11). PtdInsPs are also precursors of the water-soluble inositol phosphates (IPs), an important class of intracellular signaling molecules (reviewed in 12, 13 and 14).
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 10). These stereoisomers have been shown to be restricted to certain membranes (reviewed in 10). 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 10). 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 10). 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 10).
Phosphorylation and dephosphorylation of the inositol headgroups of PtdInsPs at specific membrane locations signals the recruitment of certain proteins essential for vesicular transport (11, and reviewed in 10). 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 10).
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 10). 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