科研之友
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摘要
Human cytomegalovirus glycoprotein B (gB) plays a role in the fusion of the virion envelope with the host cell membrane and in syncytium formation in infected cells. Hydrophobic sequences at the carboxyl terminus, amino acids (aa) 714 to 771, anchor gB in the lipid bilayer, but the unusual length of this domain suggests that it may serve another role in gB structure. To explore the s) of this region, we deleted aa 717 to 747 (gB Delta I mutation), aa 751 to 771 (gB Delta II mutation), and aa 717 to 772 (gB Delta I-II mutation) and constructed a substitution mutation, Lys-738 to Val (Lys748Val)-Asn749Ala-Pro750Ile (gB KNPm). Mutated forms of gB were expressed in U373 glioblastoma cells and subjected to analysis by flow cytometry, confocal microscopy, and immunoprecipitation. Mutations gB Delta I-II and gB Delta II alone caused secretion of gB into the medium, confirming that aa 751 to 771 function as a membrane anchor. In contrast, mutations gB Delta I and gB KNPm blocked cell surface expression and arrested gB transport in the endoplasmic reticulum (ER). Detailed examination of gB Delta I and gB KNPm with a panel of monoclonal antibodies showed that the mutated forms were indistinguishable from wild-type gB in conformation and formed oligomers; however, they remained sensitive to endoglycosidase II and did not undergo endoproteolytic cleavage. Analysis of protein complexes formed by gB and molecular chaperones in the ER showed that calnexin and calreticulin, lectin-like chaperones, bound equal amounts of uncleaved wild-type gB, gB Delta I, and gB KNPm, but the glucose-regulated proteins 78 (BiP) and 94 formed stable complexes only with the mutated forms, causing their retention in the ER. Our studies show that aa 714 to 750 are key residues in the architecture of gB molecules and that the ER chaperones, which facilitate gB folding and monitor the quality of glycoproteins, detect subtle changes in folding intermediates that are conferred by mutations in this region.
- 出版日期1996-11
