title: Maturation cleavage of the murine leukemia virus Env precursor separates the transmembrane subunits to prime it for receptor triggering.
abstract: The Env protein of murine leukemia virus matures by two cleavage events. First, cellular furin separates the receptor binding surface (SU) subunit from the fusion-active transmembrane (TM) subunit and then, in the newly assembled particle, the viral protease removes a 16-residue peptide, the R-peptide from the endodomain of the TM. Both cleavage events are required to prime the Env for receptor-triggered activation. Cryoelectron microscopy (cryo-EM) analyses have shown that the mature Env forms an open cage-like structure composed of three SU-TM complexes, where the TM subunits formed separated Env legs. Here we have studied the structure of the R-peptide precursor Env by cryo-EM. TM cleavage in Moloney murine leukemia virus was inhibited by amprenavir, and the Envs were solubilized in Triton X-100 and isolated by sedimentation in a sucrose gradient. We found that the legs of the R-peptide Env were held together by trimeric interactions at the very bottom of the Env. This suggested that the R-peptide ties the TM legs together and that this prevents the activation of the TM for fusion. The model was supported by further cryo-EM studies using an R-peptide Env mutant that was fusion-competent despite an uncleaved R-peptide. The Env legs of this mutant were found to be separated, like in the mature Env. This shows that it is the TM leg separation, normally caused by R-peptide cleavage, that primes the Env for receptor triggering.
authors.affiliation: Department of Biosciences and Nutrition,  Karolinska Institute, S-141 57 Huddinge,  Sweden.
authors.forename: Robin
authors.initials: R
authors.lastname: Löving
authors.forename: Shang-Rung
authors.initials: SR
authors.lastname: Wu
authors.forename: Mathilda
authors.initials: M
authors.lastname: Sjöberg
authors.forename: Birgitta
authors.initials: B
authors.lastname: Lindqvist
authors.forename: Henrik
authors.initials: H
authors.lastname: Garoff
mesh_terms: D020285:Cryoelectron Microscopy; D004591:Electrophoresis, Polyacrylamide Gel; D031521:Fractionation, Field Flow; D015686:Gene Products, env / Q000378:metabolism*; D009052:Leukemia Virus, Murine / Q000378:metabolism*; D008958:Models, Molecular; D008979:Moloney murine leukemia virus; D017830:Octoxynol; D011487:Protein Conformation; D011498:Protein Precursors / Q000378:metabolism*; D011499:Protein Processing, Post-Translational; D021122:Protein Subunits / Q000378:metabolism
chemical_list: D015686:Gene Products, env; D011498:Protein Precursors; D021122:Protein Subunits; D017830:Octoxynol