Vicinal relationships between the major structural proteins of murine mammary tumor virus

Abstract

Spatial relationships between the major structural proteins of murine mammary tumor virus (MuMTV), a type-13 RNA tumor virus, were investigated. Untreated virions, when analyzed by diagonal gel electrophoresis, contained two major species of disulfide-linked protein oligomers, i.e., a high-molecular-weight aggregate of p18 and a dimer composed of gp34 and p28. These complexes are termed “native crosslinked oligomers.” In addition, induced crosslinking by dithiobis succinimidyl propionate (DTSP) of vicinal proteins of whole virions or rosettes (viral membranes with attached spikes or projections) was also investigated. Diagonal gel electrophoretic analyses of DTSP-crosslinked virions revealed dimers of gp55 · gp55 and gp55 · gp34 as well as the native crosslinked oligomers. No DTSP-induced crosslinking of internal viral proteins was observed. Similar studies with rosettes indicated the formation of DTSP-induced crosslinked dimers consisting of gp55·gp55 and gp34 · gp34 in addition to gp34 · p28. From these studies, we conclude that the MuMTV projection is a homooligomer of gp55 which is directly apposed to the transmembrane protein, gp34, on the external surface. In addition, a portion of the gp34 transmembrane protein interacts internally with the major core shell protein (p28) as a native crosslinked oligomer. The latter observation most probably accounts for the eccentric location of the MuMTV nucleoid. A model which derives from these studies is discussed vis-à-vis virus assembly and previous studies of MuMTV structure.