Phosphoinositides (PIs) make up only a small fraction of cellular phospholipids, yet they control almost all aspects of a cell's life and death. These lipids gained tremendous research interest as plasma membrane signaling molecules when discovered in the 1970s and 1980s. Research in the last 15 years has added a wide range of biological processes regulated by PIs, turning these lipids into one of the most universal signaling entities in eukaryotic cells. PIs control organelle biology by regulating vesicular trafficking, but they also modulate lipid distribution and metabolism via their close relationship with lipid transfer proteins. PIs regulate ion channels, pumps, and transporters and control both endocytic and exocytic processes. The nuclear phosphoinositides have grown from being an epiphenomenon to a research area of its own. As expected from such pleiotropic regulators, derangements of phosphoinositide metabolism are responsible for a number of human diseases ranging from rare genetic disorders to the most common ones such as cancer, obesity, and diabetes. Moreover, it is increasingly evident that a number of infectious agents hijack the PI regulatory systems of host cells for their intracellular movements, replication, and assembly. As a result, PI converting enzymes began to be noticed by pharmaceutical companies as potential therapeutic targets. This review is an attempt to give an overview of this enormous research field focusing on major developments in diverse areas of basic science linked to cellular physiology and disease.

Phosphoinositides: Tiny Lipids With Giant Impact on Cell Regulation

The protein kinase Akt/PKB is activated via a multistep process by a variety of signals. In the early steps of this process, PI-3 kinase-generated D3-phosphorylated phosphoinositides bind the Akt PH domain and induce the translocation of the kinase to the plasma membrane where it co-localizes with phosphoinositide-dependent kinase-1. By binding to the PH domains of both Akt and phosphoinositide-dependent kinase-1, D3-phosphorylated phosphoinositides appear to also induce conformational changes that permit phosphoinositide-dependent kinase-1 to phosphorylate the activation loop of Akt. The paradigm of Akt activation via phosphoinositide-dependent phosphorylation provided a framework for research into the mechanism of activation of other members of the AGC kinase group (p70S6K, PKC, and PKA) and members of the Tec tyrosine kinase family (TecI, TecII, Btk/Atk, Itk/Tsk/Emt, Txk/Rlk, and Bm/Etk). The result was the discovery that these kinases and Akt are activated by overlapping pathways. In this review, we present our current understanding of the regulation and function of the Akt kinase and we discuss the common and unique features of the activation processes of Akt and the AGC and Tec kinase families. In addition, we present an overview of the biosynthesis of phosphoinositides that contribute to the regulation of these kinases.

AKT/PKB and Other D3 Phosphoinositide-Regulated Kinases: Kinase Activation by Phosphoinositide-Dependent Phosphorylation

The yeast Saccharomyces cerevisiae is a powerful experimental system to study biochemical, cell biological and molecular biological aspects of lipid synthesis. Most but not all genes encoding enzymes involved in fatty acid, phospholipid, sterol or sphingolipid biosynthesis of this unicellular eukaryote have been cloned, and many gene products have been functionally characterized. Less information is available about genes and gene products governing the transport of lipids between organelles and within membranes, turnover and degradation of complex lipids, regulation of lipid biosynthesis, and linkage of lipid metabolism to other cellular processes. Here we summarize current knowledge about lipid biosynthetic pathways in S. cerevisiae and describe the characteristic features of the gene products involved. We focus on recent discoveries in these fields and address questions on the regulation of lipid synthesis, subcellular localization of lipid biosynthetic steps, cross-talk between organelles during lipid synthesis and subcellular distribution of lipids. Finally, we discuss distinct functions of certain key lipids and their possible roles in cellular processes.

Biochemistry, Cell Biology and Molecular Biology of Lipids of Saccharomyces cerevisiae

REGULATED fusion of secretory granules with the plasma membrane in secretory cells requires ATP, Ca2+ and cytosolic1-3 as well as membrane4 proteins. ATP-dependent steps in Ca2+-activated secretion from PC 12 cells require three cytosolic PEP proteins (priming in exocytosis proteins, PEP1–3)5,6, the identity of which will provide insights into the required ATP-using reactions. PEP3 was recently identified as phosphatidylinositol transfer protein (PtdlnsTP)6, and here we report that PEP1 consists of the type I phosphatidylinositol-4-phosphate 5-kinase (PtdlnsPSK). The roles of PEP3/PtdInsTP and PEP1/ PtdlnsPSK in sequential phosphoinositide recruitment and phosphorylation explains their synergistic activity in ATP-dependent priming. Moreover, inhibition of Ca2+-activated secretion by PtdIns(4,5)P2-specific antibodies and phospholipase C implies that 5-phosphorylated inositides play a novel, necessary role in the regulated secretory pathway. The results indicate that lipid kinase-mediated phosphorylation is an important basis for ATP use in the exocytotic pathway.

Jacqueline R. Altken

Papers

Mutations in the CDP-choline pathway for phospholipid biosynthesis bypass the requirement for an essential phospholipid transfer protein.