The interface between phosphatidylinositol transfer protein function and phosphoinositide signaling in higher eukaryotes

Abstract

P 3 ]. Yeasts (Saccharomyces) synthesize all of these except PtdIns(3,4)P 2 and PtdIns(3,4,5)P 3. This cohort of molecules is produced by a series of reversible phosphorylation and dephosphorylation reactions catalyzed by phosphoinositide kinases (PIKs) and phosphatases, respectively, that target the inositol headgroup at positions C3, C4, and C5 (1, 2). These chemically distinct lipids, although small in number, control an impressively large set of intracellular activities. How is such diversification of biological function achieved? Current views for how PtdIns and phosphoinositide signaling are diversified focus on regulatory mechanisms that include: the regulation and localization of the enzymes that produce, degrade, or sequester phosphoinositides; the identities of the effector proteins that recognize these Abstract Phosphoinositides are key regulators of a large number of diverse cellular processes that include membrane trafficking, plasma membrane receptor signaling, cell proliferation, and transcription. How a small number of chemically distinct phosphoinositide signals are functionally amplified to exert specific control over such a diverse set of biological outcomes remains incompletely understood. To this end, a novel mechanism is now taking shape, and it involves phosphatidylinositol (PtdIns) transfer proteins (PITPs). The concept that PITPs exert instructive regulation of PtdIns 4-OH kinase activities and thereby channel phosphoinositide production to specific biological outcomes, identifies PITPs as central factors in the diversification of phosphoinositide signaling. There are two evolutionarily distinct families of PITPs: the Sec14-like and the StAR-related lipid transfer domain (START)-like families. Of these two families, the START-like PITPs are the least understood. Herein, we review recent insights into the biochemical, cellular, and physiological function of both PITP families with greater emphasis on the START-like PITPs, and we discuss the underlying mechanisms through which these proteins regulate phosphoinositide signaling and how these actions translate to human health and disease.-Grabon, A., V. A. Bankaitis, and M. I. McDermott. The interface between phosphatidylinositol transfer protein function and phosphoinositide signaling in higher eukaryotes.

Synthesis and Cellular Labeling of Caged Phosphatidylinositol Derivatives

“…To date, no binding domain able to specifically recognize PI has been described. However, a class I PI transport protein (PITP) is responsible for transferring PI from the ER to the plasma membrane and therefore might be considered a PI‐binding protein . For this reason, we aimed at preparing a set of tools, namely, photo‐cross‐linkable PI derivatives that are able to capture proteins binding to the lipid.…”

“…Interestingly, with increased uncaging times, cells were successively more stained at the plasma membrane, a location significantly different from the location of the intact caged lipid (Figure B). This indicates that between uncaging and photo‐cross‐linking, the lipid was transferred to the plasma membrane as was described for endogenous PI . It seems that the uncaged lipid is accepted by the major PI transfer proteins PITPα and PITPβ (see below).…”

“…However, a class I PI transport protein (PITP) is responsible for transferring PI from the ER to the plasma membrane and therefore might be considered a PI-binding protein. [4] For this reason, we aimed at preparing a set of tools, namely, photocross-linkable PI derivatives that are able to capture proteins binding to the lipid. In the past, we successfully used diazirine groups attached to the alkyl fatty acid tails.…”

“…This indicates that between uncaging and photocross-linking, the lipid was transferred to the plasma membrane as was described for endogenous PI. [4] It seems that the uncaged lipid is accepted by the major PI transfer proteins PITPa and PITPb (see below).…”

A Sec14-like Phosphatidylinositol Transfer Protein Paralog Defines a Novel Class of Heme-binding Proteins With An Unusual Heme Coordination Mechanism

“…In this way, the lipid exchange cycle renders PtdIns a superior substrate for the enzyme. Structure-based predictions that PtdIns-binding is conserved across the Sec14 superfamily, while the nature of the priming lipid is diversified, suggests a mechanism where the PITP acts as a metabolic sensor for activating PtdIns4P signaling in response to a metabolic input (Schaaf et al, 2008;Bankaitis et al, 2010;Grabon et al, 2019).As all characterized Sec14-like PITPs are lipid-binding proteins, interrogation of the priming ligand question has focused on lipids as it is widely presumed that lipid exchange/transfer activity is the common feature amongst these proteins. Recent work suggests the ligand menu for Sec14-like proteins might be broader than previously thought, however.Cavity mapping studies identified the yeast Sec14-like protein Sfh5 as exhibiting an electropositive surface and a uniquely basic 'lipid-binding' cavity -thus suggesting Sfh5 might bind a ligand that is not a lipid (Tripathi et al, 2019).…”