Cédric Montigny

Functional Properties of Sarcoplasmic Reticulum Ca2+-ATPase after Proteolytic Cleavage at Leu119-Lys120, Close to the A-domain

By measuring the phosphorylation levels of individual proteolytic fragments of SERCA1a separated by electrophoresis after their phosphorylation, we were able to study the catalytic properties of a p95C-p14N complex arising from SERCA1a cleavage by proteinase K between Leu119 and Lys120, in the loop linking the A-domain with the second transmembrane segment. ATP hydrolysis by the complex was very strongly inhibited, although ATP-dependent phosphorylation and the conversion of the ADP-sensitive E1P form to E2P still occurred at appreciable rates. However, the rate of subsequent dephosphorylation of E2P was inhibited to a dramatic extent, and this was also the case for the rate of “backdoor” formation of E2P from E2 and Pi. E2P formation from E2 at equilibrium nevertheless indicated little change in the apparent affinity for Pi or Mg2+, while binding of orthovanadate was weaker. The p95C-p14N complex also had a slightly reduced affinity for Ca2+ and exhibited a reduced rate for its Ca2+-dependent transition from E2 to Ca2E1. Thus, disruption of the N-terminal link of the A-domain with the transmembrane region seems to shift the conformational equilibria of Ca2+-ATPase from the E1/E1P toward the E2/E2P states and to increase the activation energy for dephosphorylation of Ca2+-ATPase, reviving the old idea of the A-domain being a phosphatase domain as part of the transduction machinery.

Hepatitis C Virus Envelope Glycoprotein E1 Forms Trimers at the Surface of the Virion

ABSTRACT In hepatitis C virus (HCV)-infected cells, the envelope glycoproteins E1 and E2 assemble as a heterodimer. To investigate potential changes in the oligomerization of virion-associated envelope proteins, we performed SDS-PAGE under reducing conditions but without thermal denaturation. This revealed the presence of SDS-resistant trimers of E1 in the context of cell-cultured HCV (HCVcc) as well as in the context of HCV pseudoparticles (HCVpp). The formation of E1 trimers was found to depend on the coexpression of E2. To further understand the origin of E1 trimer formation, we coexpressed in bacteria the transmembrane (TM) domains of E1 (TME1) and E2 (TME2) fused to reporter proteins and analyzed the fusion proteins by SDS-PAGE and Western blotting. As expected for strongly interacting TM domains, TME1–TME2 heterodimers resistant to SDS were observed. These analyses also revealed homodimers and homotrimers of TME1, indicating that such complexes are stable species. The N-terminal segment of TME1 exhibits a highly conserved GxxxG sequence, a motif that is well documented to be involved in intramembrane protein-protein interactions. Single or double mutations of the glycine residues (Gly354 and Gly358) in this motif markedly decreased or abrogated the formation of TME1 homotrimers in bacteria, as well as homotrimers of E1 in both HCVpp and HCVcc systems. A concomitant loss of infectivity was observed, indicating that the trimeric form of E1 is essential for virus infectivity. Taken together, these results indicate that E1E2 heterodimers form trimers on HCV particles, and they support the hypothesis that E1 could be a fusion protein. IMPORTANCE HCV glycoproteins E1 and E2 play an essential role in virus entry into liver cells as well as in virion morphogenesis. In infected cells, these two proteins form a complex in which E2 interacts with cellular receptors, whereas the function of E1 remains poorly understood. However, recent structural data suggest that E1 could be the protein responsible for the process of fusion between viral and cellular membranes. Here we investigated the oligomeric state of HCV envelope glycoproteins. We demonstrate that E1 forms functional trimers after virion assembly and that in addition to the requirement for E2, a determinant for this oligomerization is present in a conserved GxxxG motif located within the E1 transmembrane domain. Taken together, these results indicate that a rearrangement of E1E2 heterodimer complexes likely occurs during the assembly of HCV particles to yield a trimeric form of the E1E2 heterodimer. Gaining structural information on this trimer will be helpful for the design of an anti-HCV vaccine.

Coordinated Overexpression in Yeast of a P4-ATPase and Its Associated Cdc50 Subunit: The Case of the Drs2p/Cdc50p Lipid Flippase Complex

Structural and functional characterization of integral membrane proteins requires milligram amounts of purified sample. Unless the protein you are studying is abundant in native membranes, it will be critical to overexpress the protein of interest in a homologous or heterologous way, and in sufficient quantities for further purification. The situation may become even more complicated if you chose to investigate the structure and function of a complex of two or more membrane proteins. Here, we describe the overexpression of a yeast lipid flippase complex, namely the P4-ATPase Drs2p and its associated subunit Cdc50p, in a coordinated manner. Moreover, we can take advantage of the fact that P4-ATPases, like most other P-type ATPases, form an acid-stable phosphorylated intermediate, to verify that the expressed complex is functional.

Optimisation of Recombinant Production of Active Human Cardiac SERCA2a ATPase

Methods for recombinant production of eukaryotic membrane proteins, yielding sufficient quantity and quality of protein for structural biology, remain a challenge. We describe here, expression and purification optimisation of the human SERCA2a cardiac isoform of Ca2+ translocating ATPase, using Saccharomyces cerevisiae as the heterologous expression system of choice. Two different expression vectors were utilised, allowing expression of C-terminal fusion proteins with a biotinylation domain or a GFP- His8 tag. Solubilised membrane fractions containing the protein of interest were purified onto Streptavidin-Sepharose, Ni-NTA or Talon resin, depending on the fusion tag present. Biotinylated protein was detected using specific antibody directed against SERCA2 and, advantageously, GFP-His8 fusion protein was easily traced during the purification steps using in-gel fluorescence. Importantly, talon resin affinity purification proved more specific than Ni-NTA resin for the GFP-His8 tagged protein, providing better separation of oligomers present, during size exclusion chromatography. The optimised method for expression and purification of human cardiac SERCA2a reported herein, yields purified protein (> 90%) that displays a calcium-dependent thapsigargin-sensitive activity and is suitable for further biophysical, structural and physiological studies. This work provides support for the use of Saccharomyces cerevisiae as a suitable expression system for recombinant production of multi-domain eukaryotic membrane proteins.

Functional and Structural Insights into Sarcolipin, a Regulator of the Sarco-Endoplasmic Reticulum Ca 2+ -ATPases

Sarcolipin (SLN), a transmembrane peptide from sarcoplasmic reticulum, is one of the major proteins involved in the muscle contraction/relaxation process. A number of enzymological studies have underlined its regulatory role in connection with the SERCA1a activity. Indeed, SLN folds as a unique transmembrane helix and binds to SERCA1a in a groove close to transmembrane helices M2, M6, and M9, as proposed initially by cross-linking experiments and recently detailed in the 3D structures of the SLN–Ca2+-ATPase complex. In addition, association of SLN with SERCAs may depend on its phosphorylation. SLN possesses a peculiar C-terminus (RSYQY) critical for the regulation of the ATPases. This luminal tail appears to be essential for addressing SLN to the ER membrane. Moreover, we recently demonstrated that some SLN isoforms are acylated on cysteine 9, a feature which remained unnoticed so far even in the recent crystal structures of the SLN–SERCA1a complex. The removal of the fatty acid chain was shown to increase the activity of the membrane-embedded Ca2+-ATPase by about 20 %. The exact functional and structural role of this post-translational modification is presently unknown. Recent data are in favor of a key regulator role of SLN in muscle-based thermogenesis in mammals. The possible link of SLN to heat production could occur through an uncoupling of the SERCA1a-mediated ATP hydrolysis from calcium transport. Considering those particular features and the fact that SLN is not expressed at the same level in different tissues, the role of SLN and its exact mechanism of regulation remain sources of interrogation.