Mass spectrometry to characterize the binding of a peptide to a lipid surface

A Pressure-dependent Model for the Regulation of Lipoprotein Lipase by Apolipoprotein C-II

Apolipoprotein C-II (apoC-II) is the co-factor for lipoprotein lipase (LPL) at the surface of triacylglycerol-rich lipoproteins. LPL hydrolyzes triacylglycerol, which increases local surface pressure as surface area decreases and amphipathic products transiently accumulate at the lipoprotein surface. To understand how apoC-II adapts to these pressure changes, we characterized the behavior of apoC-II at multiple lipid/water interfaces. ApoC-II adsorption to a triacylglycerol/water interface resulted in large increases in surface pressure. ApoC-II was exchangeable at this interface and desorbed on interfacial compressions. These compressions increase surface pressure and mimic the action of LPL. Analysis of gradual compressions showed that apoC-II undergoes a two-step desorption, which indicates that lipid-bound apoC-II can exhibit at least two conformations. We characterized apoC-II at phospholipid/triacylglycerol/water interfaces, which more closely mimic lipoprotein surfaces. ApoC-II had a large exclusion pressure, similar to that of apoC-I and apoC-III. However, apoC-II desorbed at retention pressures higher than those seen with the other apoCs. This suggests that it is unlikely that apoC-I and apoC-III inhibit LPL via displacement of apoC-II from the lipoprotein surface. Upon rapid compressions and re-expansions, re-adsorption of apoC-II increased pressure by lower amounts than its initial adsorption. This indicates that apoC-II removed phospholipid from the interface upon desorption. These results suggest that apoC-II regulates the activity of LPL in a pressure-dependent manner. ApoC-II is provided as a component of triacylglycerol-rich lipoproteins and is the co-factor for LPL as pressure increases. Above its retention pressure, apoC-II desorbs and removes phospholipid. This triggers release of LPL from lipoproteins.

Site-directed mutagenesis of apolipoprotein CII to probe the role of its secondary structure for activation of lipoprotein lipase

Apolipoprotein CII (apoCII) is a necessary activator for lipoprotein lipase (LPL). We had identified four residues (Tyr-63, Ile-66, Asp-69, and Gln-70), presumably contained in an alpha-helix, as a potential binding site for LPL. We have now used structure prediction, mutagenesis, and functional assays to explore the functional role of the secondary structure in this part of apoCII. First, mutants were generated by replacements with proline residues to disturb the helical structure. Activation by mutant G65P was reduced by 30%, whereas mutant S54P retained activation ability. Mutants V71P and L72P should be located outside the LPL-binding site, but V71P was totally inactive, whereas activation by L72P was reduced by 65%. Insertion of alanine after Tyr-63, changing the position of the putative LPL-binding site in relation to the hydrophobic face of the alpha-helix, also severely impeded the activation ability, and a double mutant (Y63A/I66A) was completely inactive. Next, to investigate the importance of conserved hydrophobic residues in the C-terminal end of apoCII, Phe-67, Val-71, Leu-72, and Leu-75 were exchanged for polar residues. Only F67S showed dramatic loss of function. Finally, fragment 39-62, previously claimed to activate LPL, was found to be completely inactive. Our data support the view that the helical structure close to the C-terminal end of apoCII is important for activation of LPL, probably by placing residues 63, 66, 69, and 70 in an optimal steric position. The structural requirements for the hydrophobic face on the back side of this helix and further out toward the C terminus were less stringent.

Apolipoprotein C-II is a novel substrate for matrix metalloproteinases

We previously reported an efficient proteomic approach to identify matrix metalloproteinase (MMP) substrates from complex protein mixture. Using the proteomic approach, apolipoprotein C-II (apoC-II), which is a cofactor of lipoprotein lipase (LPL) and a component of very-low density lipoprotein and chylomicron, has been identified as a putative MMP-14 substrate. Cleavage of apoC-II, with various MMPs, demonstrated that apoC-II is cleaved most efficiently by MMP-14, and also by MMP-7, among the tested MMPs. The 79-amino acid residue apoC-II was cleaved between Asn35 and Leu36 by MMP-14, and between Phe14 and Leu15 and between Asn35 and Leu36 by MMP-7. Cleavage of apoC-II by MMP-14 markedly decreased LPL activity and would thus impair hydrolysis of triglycerides in plasma and transfer of fatty acids to tissues. Our result suggests that cleavage of apoC-II by MMPs would be important for development of pathophysiological situations of apoC-II deficiency such as atherosclerosis.

Phospholipid complexation and association with apolipoprotein C-II: insights from mass spectrometry

The interactions between phospholipid molecules in suspensions have been studied by using mass spectrometry. Electrospray mass spectra of homogeneous preparations formed from three different phospholipid molecules demonstrate that under certain conditions interactions between 90 and 100 lipid molecules can be preserved. In the presence of apolipoprotein C-II, a phospholipid binding protein, a series of lipid molecules and the protein were observed in complexes. The specificity of binding was demonstrated by proteolysis; the resulting mass spectra reveal lipid-bound peptides that encompass the proposed lipid-binding domain. The mass spectra of heterogeneous suspensions and their complexes with apolipoprotein C-II demonstrate that the protein binds simultaneously to two different phospholipids. Moreover, when apolipoprotein C-II is added to lipid suspensions formed with local concentrations of the same lipid molecule, the protein is capable of remodeling the distribution to form one that is closer to a statistical arrangement. These observations demonstrate a capacity for apolipoprotein C-II to change the topology of the phospholipid surface. More generally, these results highlight the fact that mass spectrometry can be used to probe lipid interactions in both homogeneous and heterogeneous suspensions and demonstrate reorganization of the distribution of lipids upon surface binding of apolipoprotein C-II.

On the structure and function of apolipoproteins: more than a family of lipid-binding proteins

Exchangeable apolipoproteins have been the subject of intense biomedical investigation for decades. However, only in recent years the elucidation of the three-dimensional structure reported for several members of the apolipoprotein family has provided insights into their functions at a molecular level for the first time. Moreover, the role of exchangeable apolipoproteins in several cellular events distinct from lipid metabolism has recently been described. This review summarizes these contributions, which have not only allowed the identification of the apolipoprotein domains that determine substrate binding specificity and/or affinity but also the plausible molecular mechanism(s) involved.

Functional analyses of human apolipoprotein CII by site-directed mutagenesis: identification of residues important for activation of lipoprotein lipase

Apolipoprotein CII (apoCII) activates lipoprotein lipase (LPL). Seven residues, located on one face of a model alpha-helix spanning residues 59-75, are fully conserved in apoCII from ten different animal species. We have mutated these residues one by one. Substitution of Ala(59) by glycine, or Thr(62) and Gly(65) by alanine did not change the activation, indicating that these residues are outside the LPL-binding site. Replacement of Tyr(63), Ile(66), Asp(69), or Gln(70) by alanine lowered the affinity for LPL and the catalytic activity of the LPL-apoCII complex. For each residue several additional replacements were made. Most mutants retained some activating ability, but replacement of Tyr(63) by phenylalanine or tryptophan and Gln(70) by glutamate caused almost complete loss of activity. All mutants bound to liposomes with similar affinity as wild-type apoCII, and they also bound with similar affinity to LPL in the absence of hydrolyzable lipids. However, the inactive mutants did not compete with wild-type apoCII in the activation assay. Therefore, we conclude that the productive apoCII-LPL interaction may be dependent on substrate molecules. In summary, our data demonstrate that residues 63, 66, 69, and 70 are of special importance for the function of apoCII, but no single amino acid residue is absolutely crucial.

Membrane interactions of a novel viral enterotoxin: rotavirus nonstructural glycoprotein NSP4

The rotavirus enterotoxin, NSP4, is a novel secretory agonist that also plays a role in the unique rotavirus morphogenesis that involves a transient budding of newly made immature viral particles into the endoplasmic reticulum. NSP4 and an active peptide corresponding to NSP4 residues 114 to 135 (NSP4(114-135)) mobilize intracellular calcium and induce secretory chloride currents when added exogenously to intestinal cells or mucosa. Membrane-NSP4 interactions may contribute to these alterations; however, details of a lipid-binding domain are unresolved. Therefore, circular dichroism was used to determine (i) the interaction(s) of NSP4 and NSP4(114-135) with model membranes, (ii) the conformational changes elicited in NSP4 upon interacting with membranes, (iii) if NSP4(114-135) is a membrane interacting domain, and (iv) the molar dissociation constant (K(d)) of NSP4(114-135) with defined lipid vesicles. Circular dichroism revealed for the first time that NSP4 and NSP4(114-135) undergo secondary structural changes upon interaction with membrane vesicles. This interaction was highly dependent on both the membrane surface curvature and the lipid composition. NSP4 and NSP4(114-135) preferentially interacted with highly curved, small unilamellar vesicle membranes (SUV), but significantly less with low-curvature, large unilamellar vesicle membranes (LUV). Binding to SUV, but not LUV, was greatly enhanced by negatively charged phospholipids. Increasing the SUV cholesterol content, concomitant with the presence of negatively charged phospholipids, further potentiated the interaction of NSP4(114-135) with the SUV membrane. The K(d) of NSP4(114-135) was determined as well as partitioning of NSP4(114-135) with SUVs in a filtration-binding assay. These data confirmed NSP4 and its active peptide interact with model membranes that mimic caveolae.