Functional analyses of phosphatidylserine/PI(4)P exchangers with diverse lipid species and membrane contexts reveal unanticipated rules on lipid transfer

Abstract

Background Lipid species are accurately distributed in the eukaryotic cell so that organelle and plasma membranes have an adequate lipid composition to support numerous cellular functions. In the plasma membrane, a precise regulation of the level of lipids such as phosphatidylserine, PI(4)P, and PI(4,5)P2, is critical for maintaining the signaling competence of the cell. Several lipid transfer proteins of the ORP/Osh family contribute to this fine-tuning by delivering PS, synthesized in the endoplasmic reticulum, to the plasma membrane in exchange for PI(4)P. To get insights into the role of these PS/PI(4)P exchangers in regulating plasma membrane features, we question how they selectively recognize and transfer lipid ligands with different acyl chains, whether these proteins exchange PS exclusively for PI(4)P or additionally for PI(4,5)P2, and how sterol abundance in the plasma membrane impacts their activity. Results We measured in vitro how the yeast Osh6p and human ORP8 transported PS and PI(4)P subspecies of diverse length and unsaturation degree between membranes by fluorescence-based assays. We established that the exchange activity of Osh6p and ORP8 strongly depends on whether these ligands are saturated or not, and is high with representative cellular PS and PI(4)P subspecies. Unexpectedly, we found that the speed at which these proteins individually transfer lipid ligands between membranes is inversely related to their affinity for them and that high-affinity ligands must be exchanged to be transferred more rapidly. Next we determined that Osh6p and ORP8 cannot use PI(4,5)P2 for exchange processes, because it is a low-affinity ligand, and do not transfer more PS into sterol-rich membranes. Conclusions Our study provides new insights into PS/PI(4)P exchangers by indicating the degree to which they can regulate the acyl chain composition of the PM, and how they control PM phosphoinositide levels. Moreover, we establish general rules on how the activity of lipid transfer proteins relates to their affinity for ligands.

Quantitative Models of Lipid Transfer and Membrane Contact Formation

“…Otherwise, a lipid with an excessive binding affinity for the LTP would be trapped in the binding cavity and block the exchange reaction. Consistent with this view, the LTP-lipid binding affinity appears to be tuned to a similar level for efficient exchange of the cognate lipids (Ikhlef et al ., 2021). Therefore, based on the energetics of lipid extraction and solubilization, we hypothesize that shuttle LTPs are generally lipid exchangers.…”

“…Our model of lipid exchange differs from the previous model developed by Drin and coworkers (von Filseck et al ., 2015b; Ikhlef et al ., 2021), in which several intermediate states of Osh4p-mediated lipid transfer are included, for example, the states in which Osh4p is bound to the membrane but not loaded with any lipid. Consequently, the model contains six coupled reactions with more than 10 rate constants, which make it difficult to derive an analytical solution for the lipid transfer rate and fit the model to experimental data.…”

“…The LTP-mediated lipid exchange depends on the curvature and composition of membranes (von Filseck et al ., 2015b; Ikhlef et al ., 2021). For example, Osh4p-PI4P extracts DHE more rapidly in DOPC liposomes than in POPC liposomes.…”

“…von Filseck et al, 2015b;Ikhlef et al, 2021). For example, Osh4p-PI4P extracts DHE more rapidly in DOPC liposomes than in POPC liposomes.…”

Phosphatidylserine in Diabetes Research

“…Moreover, the conversion of PS to PI4P depends on lipid transfer proteins (LTPs), and PS species affect LTP activity. Experimental results indicated that one double bond in either PS acyl chain was enough to markedly change the rate of PS conversion into PI4P, 48 showing that PS species distribution can modify insulin signal transduction. In addition, via its C1q domain, adiponectin binds to PS and sphingolipids, which may be related to dead cell clearance.…”