Pablo R. Castello

Effects of phosphatidylethanolamine glycation on lipid-protein interactions and membrane protein thermal stability

Non-enzymatic glycation of biomolecules has been implicated in the pathophysiology of aging and diabetes. Among the potential targets for glycation are biological membranes, characterized by a complex organization of lipids and proteins interacting and forming domains of different size and stability. In the present study, we analyse the effects of glycation on the interactions between membrane proteins and lipids. The phospholipid affinity for the transmembrane surface of the PMCA (plasma-membrane Ca(2+)-ATPase) was determined after incubating the protein or the phospholipids with glucose. Results show that the affinity between PMCA and the surrounding phospholipids decreases significantly after phosphospholipid glycation, but remains unmodified after glycation of the protein. Furthermore, phosphatidylethanolamine glycation decreases by approximately 30% the stability of PMCA against thermal denaturation, suggesting that glycated aminophospholipids induce a structural rearrangement in the protein that makes it more sensitive to thermal unfolding. We also verified that lipid glycation decreases the affinity of lipids for two other membrane proteins, suggesting that this effect might be common to membrane proteins. Extending these results to the in vivo situation, we can hypothesize that, under hyperglycaemic conditions, glycation of membrane lipids may cause a significant change in the structure and stability of membrane proteins, which may affect the normal functioning of membranes and therefore of cells.

Thermal stability of the plasma membrane calcium pump. Quantitative analysis of its dependence on lipid-protein interactions.

Abstract. Thermal stability of plasma membrane Ca2+ pump was systematically studied in three micellar systems of different composition, and related with the interactions amphiphile-protein measured by fluorescence resonance energy transfer. Thermal denaturation was characterized as an irreversible process that is well described by a first order kinetic with an activation energy of 222 ± 12 kJ/mol in the range 33–45°C. Upon increasing the mole fraction of phospholipid in the mixed micelles where the Ca2+ pump was reconstituted, the kinetic coefficient for the inactivation process diminished until it reached a constant value, different for each phospholipid species. We propose a model in which thermal stability of the pump depends on the composition of the amphiphile monolayer directly in contact with the transmembrane protein surface. Application of this model shows that the maximal pump stability is attained when 80% of this surface is covered by phospholipids. This analysis provides an indirect measure of the relative affinity phospholipid/detergent for the hydrophobic transmembrane surface of the protein (KLD) showing that those phospholipids with higher affinity provide greater stability to the Ca2+ pump. We developed a method for directly measure KLD by using fluorescence resonance energy transfer from the membrane protein tryptophan residues to a pyrene-labeled phospholipid. KLD values obtained by this procedure agree with those obtained from the model, providing a strong evidence to support its validity.

MOLECULAR CHARACTERIZATION OF THE GLYCATED PLASMA MEMBRANE CALCIUM PUMP

Abstract. We have previously demonstrated (Diabetes39:707–711, 1990) that in vitro glycation of the red cell Ca2+ pump diminishes the Ca2+-ATPase activity of the enzyme up to 50%. Such effect is due to the reaction of glucose with lysine residues of the Ca2+ pump (Biochem. J.293:369–375, 1993). The aim of this work was to determine whether the effect of glucose is due to a full inactivation of a fraction of the total population of Ca2+ pump, or to a partial inactivation of all the molecules. Glycation decreased the Vmax for the ATPase activity leaving unaffected the apparent affinities for Ca2+, calmodulin or ATP. The apparent turnover was identical in both, the glycated and the native enzyme. Glycation decreased the Vmax for the ATP-dependent but not for the calmodulin-activated phosphatase activities. Concomitantly with the inhibition, up to 6.5% of the lysine residues were randomly glycated. The probabilistic analysis of the relation between the enzyme activity and the fraction of nonmodified residues indicates that only one Lys residue is responsible for the inhibition. We suggest that glucose decreases the Ca2+-ATPase activity by reacting with one essential Lys residue probably located in the vicinity of the catalytic site, which results in the full inactivation of the enzyme. Thus, Ca2+-ATPase activity measured in erythrocyte membranes or purified enzyme preparations preincubated with glucose depends on the remaining enzyme molecules in which the essential Lys residue stays unglycated.

Oligomerization of the plasma membrane calcium pump involves two regions with different thermal stability

Ca2+ pump dimerization was studied by using a combined approach of thermal denaturation and fluorescence resonance energy transfer. The measurement of calcium pump ability to dimerize after the unfolding of individual functional domains of the enzyme demonstrated the existence of two different regions involved in the self‐association process. One of these regions is highly susceptible to thermal unfolding and was identified as the calmodulin (CaM)‐binding domain. The other region whose thermal stability is higher than those of the catalytic and CaM‐binding domains could be related with the previously found C28W‐binding regions.

Stoichiometry of lipid–protein interaction assessed by hydrophobic photolabeling

Here we undertook a comparative study of the composition of the lipid annulus of three ATPases pertaining to the P‐type family: plasma membrane calcium pump (PMCA), sarcoplasmic reticulum calcium pump (SERCA) and Na,K‐ATPase. The photoactivatable phosphatidylcholine analogue [125I]TID‐PC/16 was incorporated into mixtures of dimyristoyl phosphatidylcholine (DMPC) and each enzyme with the aid of the nonionic detergent C12E10. After photolysis, the extent of the labeling reaction was assessed to determine the lipid:protein stoichiometry: 17 for PMCA, 18 for SERCA, 24 for the Na,K‐ATPase (α‐subunit) and 5.6 mol PC/mol protein for the Na,K‐ATPase (β‐subunit).

Quantitative analysis of membrane protein-amphiphile interactions using resonance energy transfer.

This work describes a simple method for determining the association constant of amphiphiles to membrane proteins. The method uses a fluorescent phospholipid probe, which senses the competition among unlabeled amphiphiles for positions on the transmembrane surface of the protein. The contact between the probe and the protein surface is detected through resonance energy transfer. We have analyzed theoretically this process deriving a general equation for the dependence of the energy transfer efficiency on the composition of the micelles/bilayers in which the protein is inserted. This equation includes an exchange constant for each amphiphile, which gives a measure of its affinity for the protein with respect to that of an amphiphile set as the reference. We applied this method to determine the exchange constant of different phospholipids for the plasma membrane calcium pump.

Ice-induced partial unfolding and aggregation of an integral membrane protein

Although the deleterious effects of ice on water-soluble proteins are well established, little is known about the freeze stability of membrane proteins. Here we explore this issue through a combined kinetic and spectroscopic approach using micellar-purified plasma membrane calcium pump as a model. The ATPase activity of this protein significantly diminished after freezing using a slow-cooling procedure, with the decrease in the activity being an exponential function of the storage time at 253K, with t(½)=3.9±0.6h. On the contrary, no significant changes on enzyme activity were detected when a fast cooling procedure was performed. Regardless of the cooling rate, successive freeze-thaw cycles produced an exponential decrease in the Ca(2+)-ATPase activity, with the number of cycles at which the activity was reduced to half being 9.2±0.3 (fast cooling) and 3.7±0.2 (slow cooling). PAGE analysis showed that neither degradation nor formation of SDS-stable aggregates of the protein takes place during protein inactivation. Instead, the inactivation process was found to be associated with the irreversible partial unfolding of the polypeptide chain, as assessed by Trp fluorescence, far UV circular dichroism, and 1-anilino-naphtalene-8-sulfonate binding. This inactive protein undergoes, in a later stage, a further irreversible transformation leading to large aggregates.

Phospholipid distribution around the plasma membrane calcium pump: a hydrophobic photolabeling study.

The functions of membrane proteins are highly dependent on their phospholipid environment. In this article, we have used a hydrophobic photolabeling method to study the noncovalent interactions between plasma membrane calcium pump (PMCA) and surrounding phospholipids. With this approach, we determined (1) the number of lipid molecules in close contact with the transmembrane surface, i.e., the lipid-protein stoichiometry, and (2) the distribution of lipid molecules among different regions of the protein. PMCA was photolabeled in mixed micelles containing detergent, the phosphatidylcholine photoactivatable analog 1-palmitoyl-2-[9-[2′-[125I]iodo-4′-(trifluoromethyldiazirinyl)-benzyloxycarbonyl]-nonaoyl]-sn-glycero-3-phosphocholine and different amounts of dimyristoyl phosphatidylcholine (PC). The stoichiometry was estimated after the extent of the labeling reaction had been independently assessed. We determined a maximum number of 17±1 molecules of PC in close contact with the transmembrane surface per PMCA molecule. In addition, a semiquantitative description of the phospholipid environment around different regions of PMCA was carried out after limited proteolysis of the photolabeled protein. The distribution of labels among the N-terminal (1-322), the central (323–660), and the C-terminal (661–1205) regions was 26, 36, and 38%, respectively.