SUMMARY PARAGRAPH for VPS34
VPS34 encodes a class III phosphatidylinositol (PtdIns) 3-kinase that phosphorylates phosphatidylinositol at the 3' hydroxyl position (D-3 position of the inositol ring) to produce PtdIns 3-phosphate (PtdIns 3-P) (13, 12). VPS34 was originally identified as a vacuolar protein targeting (VPT29) mutant (1), but is required for multiple protein and membrane trafficking events. These include efficient localization of a variety of vacuolar proteins (9), vacuole segregation (14), endocytosis, multivesicular body formation, constitutive autophagy via the cytoplasm-to-vacuole targeting (Cvt) pathway, and starvation-induced macroautophagy (3, 15, 16, 17, 18, 19).
Vps34p is recruited from the cytosol to the Golgi/endosome through interaction with the membrane-associated protein kinase Vps15p, which also stimulates the PtdIns 3-kinase activity of Vps34p (20, 10, 21). Interaction between Vps34p and Vps15p requires a short 28 residue-element near the C-terminus of Vps34p and two separate regions of Vps15p: the N-terminal protein kinase domain and a set of three tandem repeats of about 39 amino acids each (22). These repeat elements in Vps15p are similar to the HEAT repeats implicated in protein-protein interactions in other proteins, including Huntingtin, EF3, and Tor1p (22).
The Vps34p-Vps15p heterodimer is a component of at least two different multimeric complexes (complexes I and II), each containing Vps30p, as well as other distinct subunits that function in separate membrane trafficking processes (15). Complex I includes unique subunit Atg14p, and complex II includes Vps38p (23). Complexes I and II act in different biological processes by localizing to specific cellular locations in a manner mediated by the unique subunits of each complex (24). Complex I functions primarily in autophagy and localizes to the vacuolar membrane and the perivacuolar pre-autophagosomal structure (PAS), while complex II functions in vacuolar protein sorting and is targeted to cisternae of the late Golgi apparatus, late endosomal structures and vacuolar membranes (15, 23, 24).
Vps34p and Vps15p are also required for signalling by Gpa1p, the GTP-binding alpha subunit of the heterotrimeric G protein, at the endosome (12, 25). Gpa1p colocalizes with Vps34p and Vps15p, and binds both proteins directly in a guanine nucleotide-dependent manner (12, 25). Activated GTP-bound Gpa1p binds Vps34p, and inactive GDP-bound Gpa1p binds Vps15p (12, 25). These interactions lead to elevated production of PtdIns 3-P and promote translocation of the PtdIns 3-P binding protein Bem1p to endosomes (12, 25).
vps34 null mutants exhibit defects in protein sorting (9, 14, 26) and autophagy (15), increased sensitivity to ethanol (27, 28), and are fully defective in Gpa1p signaling, which leads to diminished efficacy and potency of mating pheromone, likely as the result of a global defect in membrane trafficking (12). Mutation of VPS34 also results in temperature-sensitive growth and defective partitioning of the vacuolar compartment between mother and daughter cells during cell division (9, 3). Cells expressing vps34-N736K do not accumulate autophagic bodies (19).
Class III phosphatidylinositol 3-kinases are conserved from yeast to human (13). The human Vps34p homolog is also found in a complex with p150, a human homolog of yeast Vps15p, which stimulates its PtdIns 3-kinase activity (3, 29). Homologs of VPS34 have been identified in other yeasts, including Candida albicans (30), Hansenula polymorpha (31) and Schizosaccharomyces pombe (32, 33). CaVps34p activity is required for C. albicans pathogenesis; C. albicans cells harboring a null allele or expressing a catalytically-inactive CaVps34 are avirulent in a mouse model for systemic candidiasis (34).
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 18 and 35). PtdInsPs are also precursors of the water-soluble inositol phosphates (IPs), an important class of intracellular signaling molecules (reviewed in 36, 37 and 38).
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 18). These stereoisomers have been shown to be restricted to certain membranes (reviewed in 18). 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 18). 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 18). 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 18).
Phosphorylation and dephosphorylation of the inositol headgroups of PtdInsPs at specific membrane locations signals the recruitment of certain proteins essential for vesicular transport (35, and reviewed in 18). 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 18).
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 18). 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