Structural Flexibility of the Pentameric SARS Coronavirus Envelope Protein Ion Channel

Abstract

Coronaviruses contain a small envelope membrane protein with cation-selective ion channel activity mediated by its transmembrane domain (ETM). In a computational study, we proposed that ion channel activity can be explained by either of two similar ETM homopentameric transmembrane alpha-helical bundles, related by a approximately 50 degrees rotation of the helices. Later, we tested this prediction, using site-specific infrared dichroism of a lysine-flanked isotopically labeled ETM peptide from the virus responsible for the severe acute respiratory syndrome, SARS, reconstituted in lipid bilayers. However, the data were consistent with the presence of a kink at the center of the ETM alpha-helix, and it did not fit completely either computational model. Herein, we have used native ETM, without flanking lysines, and show that the helix orientation is now consistent with one of the predicted models. ETM only produced one oligomeric form, pentamers, in the lipid-mimic detergent dodecylphosphocholine and in perfluorooctanoic acid. We thus report the correct backbone model for the pentameric alpha-helical bundle of ETM. The disruptive effects caused by terminal lysines probably highlight the conformational flexibility required during ion channel function.

α-Helical transmembrane peptides: A “Divide and Conquer” approach to membrane proteins

“…A C-terminal pentalysine tag, for instance, completely abolished channel activity of GlyR TM helix 2 (Tomich et al, 1998). More examples of effects of flanking sequences on the properties of TM peptides have been reported for peptides corresponding to TM sequences of GpA , major coating protein 13 (MCP13; Melnyk et al, 2003), fibroblast growth factor receptor (FGFR3; Iwamoto et al, 2005;Merzlyakov et al, 2006), protein E from Coronaviridae (SCoVE; Parthasarathy et al, 2008), and pheromone ␣-factor receptor from Saccharomyces cerevisiae (Ste2p; Naider et al, 2005;Cano-Sanchez et al, 2006).…”

The Coronavirus E Protein: Assembly and Beyond

“…This possibility was originally addressed using computational modeling, which predicted that the E protein HD could form stable dimers, trimers, and pentamers [62]. This prediction was further bolstered by several spectroscopic studies, which showed that the HD of SARS-CoV E forms a pentamer [63,64,65]. However, this result has not been confirmed for the other E proteins, or for the full length SARS-CoV E protein in a natural membrane.…”

Structural and Functional Aspects of the Small Hydrophobic (SH) Protein in the Human Respiratory Syncytial Virus

“…Indeed, viral ion channels display certain degree of structural flexibility, as seen in the influenza A virus M2 protein (Li et al, 2007) and SARS-CoV E protein (Parthasarathy et al, 2008).…”