Synthetic null-cysteine phospholamban analogue and the corresponding transmembrane domain inhibit the Ca-ATPase

Solid-state NMR and rigid body molecular dynamics to determine domain orientations of monomeric phospholamban

Solid-state NMR spectroscopy, in conjunction with rigid body molecular dynamics calculations, shows that monomeric phospholamban in lipid bilayers has two distinct helical domains, with an interhelical angle within 60-100 degrees, ruling out the possibility of a continuous alpha-helical structure for this protein.

Transmembrane domain mediated self-assembly of major coat protein subunits from Ff bacteriophage

The 50-residue major coat protein (MCP) of Ff bacteriophage exists as a single-spanning membrane protein in the Escherichia coli host inner membrane prior to assembly into lipid-free virions. Here, the molecular bases for the specificity and stoichiometry that govern the protein-protein interactions of MCP in the host membrane are investigated in detergent micelles. To address these structural issues, as well as to circumvent viability requirements in mutants of the intact protein, peptides corresponding to the effective alpha-helical TM segment of wild-type and mutant bacteriophage MCPs were synthesized. Fluorescence resonance energy transfer (FRET) experiments on the dansyl and dabcyl-labeled MCP TM domain peptides in detergent micelles demonstrated that the peptides specifically associate into non-covalent homodimers, as postulated for the biologically relevant membrane-embedded MCP oligomer. MCP peptides labeled with short-range pyrene fluorophores at the N terminus displayed excimer fluorescence consistent with homodimerization occurring in a parallel fashion. Variant peptides synthesized with single substitutions at helix-interactive positions displayed a wide range of dimer/monomer ratios on SDS-PAGE gels, which are interpreted in terms of steric volume, presence or absence of beta-branching, and the effect of polar substituents. The overall results indicate discrete roles for helix-helix interfacial residues as packing recognition elements in the membrane-inserted state, and suggest a possible correlation between phage viability and efficacy of MCP TM-TM interactions.

Role of cysteine residues in structural stability and function of a transmembrane helix bundle

To study the structural and functional roles of the cysteine residues at positions 36, 41, and 46 in the transmembrane domain of phospholamban (PLB), we have used Fmoc (N-(9-fluorenyl)methoxycarbonyl) solid-phase peptide synthesis to prepare alpha-amino-n-butyric acid (Abu)-PLB, the analogue in which all three cysteine residues are replaced by Abu. Whereas previous studies have shown that replacement of the three Cys residues by Ala (producing Ala-PLB) greatly destabilizes the pentameric structure, we hypothesized that replacement of Cys with Abu, which is isosteric to Cys, might preserve the pentameric stability. Therefore, we compared the oligomeric structure (from SDS-polyacrylamide gel electrophoresis) and function (inhibition of the Ca-ATPase in reconstituted membranes) of Abu-PLB with those of synthetic wild-type PLB and Ala-PLB. Molecular modeling provides structural and energetic insight into the different oligomeric stabilities of these molecules. We conclude that 1) the Cys residues of PLB are not necessary for pentamer formation or inhibitory function; 2) the steric properties of cysteine residues in the PLB transmembrane domain contribute substantially to pentameric stability, whereas the polar or chemical properties of the sulfhydryl group play only a minor role; 3) the functional potency of these PLB variants does not correlate with oligomeric stability; and 4) acetylation of the N-terminal methionine has neither a functional nor a structural effect in full-length PLB.

Sarcolipin, the shorter homologue of phospholamban, forms oligomeric structures in detergent micelles and in liposomes

The human 31-amino acid integral membrane protein sarcolipin (SLN), which regulates the sarcoplasmic reticulum Ca-ATPase in fast-twitch skeletal muscle, was chemically synthesized. Appropriate synthesis and purification strategies were used to achieve high purity and satisfactory yields of this hydrophobic and poorly soluble protein. Structural and functional properties of SLN were analyzed and compared with the homologous region of human phospholamban (PLB) comprising residues Ala(24)-Leu(52) (PLB-(24-52)), the regulatory protein of the cardiac sarcoplasmic reticulum Ca-ATPase. Circular dichroism spectroscopy showed that SLN is a predominantly alpha-helical protein and that the secondary structure is highly resistant to SDS and thermal denaturation. In this respect SLN is remarkably similar to PLB-(24-52). However, SLN is monomeric in SDS gels, whereas PLB-(24-52) shows a monomer-pentamer equilibrium typical for native PLB. Analytical ultracentrifugation experiments revealed that SLN oligomerizes in the presence of the nonionic detergents octylpolyoxyethylene and octyl glucoside in a concentration-dependent manner. No plateau was observed, and a pentameric state was only reached at much higher protein concentrations compared with PLB-(24-52). Chemical cross-linking showed that also in liposomes SLN has the ability to self-associate to oligomers. PLB-(24-52) specifically oligomerized to pentamers in the presence of octylpolyoxyethylene as well as in liposomes at low protein concentrations. In the presence of octylpolyoxyethylene pentamers were the main oligomeric species, whereas in liposomes monomers and dimers were predominant. Increasing the protein concentration led to self-association of PLB-(24-52) pentamers in the presence of octylpolyoxyethylene. Functional reconstitution of Ca-ATPase with PLB-(24-52) and SLN in liposomes showed that both proteins regulate the Ca-ATPase in a similar manner.