Regulation of Membrane Calcium Transport Proteins by the Surrounding Lipid Environment

Calcium ions (Ca2+) are major messengers in cell signaling, impacting nearly every aspect of cellular life. Those signals are generated within a wide spatial and temporal range through a large variety of Ca2+ channels, pumps, and exchangers. More and more evidences suggest that Ca2+ exchanges are regulated by their surrounding lipid environment. In this review, we point out the technical challenges that are currently being overcome and those that still need to be defeated to analyze the Ca2+ transport protein–lipid interactions. We then provide evidences for the modulation of Ca2+ transport proteins by lipids, including cholesterol, acidic phospholipids, sphingolipids, and their metabolites. We also integrate documented mechanisms involved in the regulation of Ca2+ transport proteins by the lipid environment. Those include: (i) Direct interaction inside the protein with non-annular lipids; (ii) close interaction with the first shell of annular lipids; (iii) regulation of membrane biophysical properties (e.g., membrane lipid packing, thickness, and curvature) directly around the protein through annular lipids; and (iv) gathering and downstream signaling of several proteins inside lipid domains. We finally discuss recent reports supporting the related alteration of Ca2+ and lipids in different pathophysiological events and the possibility to target lipids in Ca2+-related diseases.

Distinct specific interactions of the UapA transporter with membrane lipids are critical for dimerization, ER-exit and function

Transporters are transmembrane proteins that mediate the selective translocation of solutes across biological membranes. Recently, we have shown that specific interactions with plasma membrane phospholipids are essential for formation and/or stability of functional dimers of the purine transporter, UapA, a prototypic eukaryotic member of the ubiquitous NAT family. Here, we show that distinct interactions of UapA with specific or annular lipids are essential for ab initio formation of functional dimers in the ER or ER-exit and further subcellular trafficking. Through genetic screens we identify mutations that restore defects in dimer formation and/or trafficking. Suppressors of defective dimerization restore ab initio formation of UapA dimers in the ER. Most of these suppressors are located in the movable core domain, but also in the core-dimerization interface and in residues of the dimerization domain exposed to lipids. Molecular Dynamics suggest the majority of suppressors stabilize interhelical interactions in the core domain and thus assist the formation of functional UapA dimers. Among suppressors restoring dimerization, a specific mutation, T401P, was also isolated independently as a suppressor restoring trafficking, suggesting that stabilization of the core domain restores function by sustaining structural defects caused by abolishment of essential interactions with specific or annular lipids. Importantly, introduction of mutations topologically equivalent to T401P into a rat homologue of UapA, namely rSNBT1, permitted the functional expression of a mammalian NAT in A. nidulans. Thus, our results provide a potential route for the functional expression and manipulation of mammalian transporters in the model Aspergillus system.Author SummaryTransporters are proteins found in biological membranes, where they are involved in the selective movement of nutrients, ions, drugs and other small molecules across membranes. Consequently, their function is essential for cell viability, while their malfunction often results to disease. Recent findings have suggested that transporter functioning depends on proper interactions with associated membrane lipids. In this article, using UapA, a very well-studied transporter from a model fungus (Aspergillus nidulans), we show that two types of specific interactions with lipids are essential for tight and specific association of two UapA molecules in a single functional unit (UapA dimer), and for targeting to the cell membrane and transport activity. The first type of interaction concerns specific lipids associating with positively charged amino acids at the interface of the UapA dimer, whereas the other type involves lipids that interact with charged amino acids at the outer shell of the transporter. Most interestingly, defects due to abolishment of UapA-lipid interactions were shown to be restored by mutations that increase UapA stability. Using this information, we genetically manipulated and increased the stability of a mammalian transporter (rSNBT1), and thus achieved its functional expression in the experimentally tractable system of A. nidulans.

Autoinhibition mechanism of the plasma membrane calcium pump isoforms 2 and 4 studied through lipid–protein interaction

The autoinhibition/activation of the PMCA (plasma membrane Ca2+-ATPase) involves conformational changes in the membrane region of the protein that affect the amount of lipids directly associated with the transmembrane domain. The lipid–protein-dependence of PMCA isoforms 2 and 4 expressed and obtained in purified form from Saccharomyces cerevisiae was investigated using the phosphatidylcholine analogue [125I]TID-PC/16 {l-O-hexadecanoyl-2-O-[9-[[[2-[125I]iodo-4-(trifluoromemyl-3H-diazirin-3-yl)benzyl]oxy]carbonyl]nonanoyl]-sn-glycero-3-phosphocholine}, which was incorporated into mixtures of dimyristoylphosphatidylcholine and the non-ionic detergent C12E10 [deca(ethylene glycol) dodecyl ether]. We found no differences between the recombinant PMCA4 and PMCA purified from erythrocytes (ePMCA). However, titration of the half-maximal activation by Ca2+/calmodulin of PMCA2 showed 30-fold higher affinity than PMCA4. PMCA2 exhibited a lower level of labelling in the autoinhibited conformation relative to PMCA4, indicating that the lower autoinhibition was correlated with a lower exposure to lipids in the autoinhibited state. Analysis of the lipid–protein stoichiometry showed that the lipid annulus of PMCA varies: (i) in accordance to the conformational state of the enzyme; and (ii) depending on the different isoforms of PMCA. PMCA2 during Ca2+ transport changes its conformation to a lesser extent than PMCA4, an isoform more sensitive to modulation by calmodulin and acidic phospholipids. This is the first demonstration of a dynamic behaviour of annular lipids and PMCA.

Lipid–protein interactions

Intrinsic membrane proteins are solvated by a shell of lipid molecules interacting with the membrane-penetrating surface of the protein; these lipid molecules are referred to as annular lipids. Lipid molecules are also found bound between transmembrane α-helices; these are referred to as non-annular lipids. Annular lipid binding constants depend on fatty acyl chain length, but the dependence is less than expected from models based on distortion of the lipid bilayer alone. This suggests that hydrophobic matching between a membrane protein and the surrounding lipid bilayer involves some distortion of the transmembrane α-helical bundle found in most membrane proteins, explaining the importance of bilayer thickness for membrane protein function. Annular lipid binding constants also depend on the structure of the polar headgroup region of the lipid, and hotspots for binding anionic lipids have been detected on some membrane proteins; binding of anionic lipid molecules to these hotspots can be functionally important. Binding of anionic lipids to non-annular sites on membrane proteins such as the potassium channel KcsA can also be important for function. It is argued that the packing preferences of the membrane-spanning α-helices in a membrane protein result in a structure that matches nicely with that of the surrounding lipid bilayer, so that lipid and protein can meet without either having to change very much.