Polyphosphoinositide-Binding Domains: Insights from Peripheral Membrane and Lipid-Transfer Proteins

Within eukaryotic cells, biochemical reactions need to be organized on the surface of membrane compartments that use distinct lipid constituents to dynamically modulate the functions of integral proteins or influence the selective recruitment of peripheral membrane effectors. As a result of these complex interactions, a variety of human pathologies can be traced back to improper communication between proteins and membrane surfaces; either due to mutations that directly alter protein structure or as a result of changes in membrane lipid composition. Among the known structural lipids found in cellular membranes, phosphatidylinositol (PtdIns) is unique in that it also serves as the membrane-anchored precursor of low-abundance regulatory lipids, the polyphosphoinositides (PPIn), which have restricted distributions within specific subcellular compartments. The ability of PPIn lipids to function as signaling platforms relies on both non-specific electrostatic interactions and the selective stereospecific recognition of PPIn headgroups by specialized protein folds. In this chapter, we will attempt to summarize the structural diversity of modular PPIn-interacting domains that facilitate the reversible recruitment and conformational regulation of peripheral membrane proteins. Outside of protein folds capable of capturing PPIn headgroups at the membrane interface, recent studies detailing the selective binding and bilayer extraction of PPIn species by unique functional domains within specific families of lipid-transfer proteins will also be highlighted. Overall, this overview will help to outline the fundamental physiochemical mechanisms that facilitate localized interactions between PPIn lipids and the wide-variety of PPIn-binding proteins that are essential for the coordinate regulation of cellular metabolism and membrane dynamics.

The orchestra of lipid-transfer proteins at the crossroads between metabolism and signaling

Within the eukaryotic cell, more than 1000 species of lipids define a series of membranes essential for cell function. Tightly controlled systems of lipid transport underlie the proper spatiotemporal distribution of membrane lipids, the coordination of spatially separated lipid metabolic pathways, and lipid signaling mediated by soluble proteins that may be localized some distance away from membranes. Alongside the well-established vesicular transport of lipids, non-vesicular transport mediated by a group of proteins referred to as lipid-transfer proteins (LTPs) is emerging as a key mechanism of lipid transport in a broad range of biological processes. More than a hundred LTPs exist in humans and these can be divided into at least ten protein families. LTPs are widely distributed in tissues, organelles and membrane contact sites (MCSs), as well as in the extracellular space. They all possess a soluble and globular domain that encapsulates a lipid monomer and they specifically bind and transport a wide range of lipids. Here, we present the most recent discoveries in the functions and physiological roles of LTPs, which have expanded the playground of lipids into the aqueous spaces of cells.