Tryptic peptide maps from more than 50 isolates of murine leukemia virus (MuLV) have shown that, in general, the structure of core protein p30 is highly conserved. However, a structurally variable region of p30 has been identified that is functionally associated with Fv-1 tropism. On the basis of this structural variability, MuLV strains can be classified as B-tropic, N-tropic, xenotropic, and/
MATERIALS AND METHODSThe sources of the MuLV strains utilized in this study are listed in Table 1. Viruses were concentrated and purified from tissue culture media obtained from the third and fourth subcultures by differential centrifugation and banding in 15-50% sucrose gradients, as previously described (11, 13). Virus with a buoyant density of 1.16 g/cm3 was collected and the viral proteins were separated by sodium dodecyl sulfate/polyacrylamide gel electrophoresis (24). Tryptic peptide maps were derived as previously described (1, 13).
RESULTSWe have made two-dimensional tryptic peptide maps of the p30 from more than 50 strains of ecotropic, amphotropic, and xenotropic MuLVs isolated from numerous strains of mice. As expected from the NH2-terminal amino acid sequence homology (25), all of the MuLV p3Os displayed a remarkable structural similarity. This indicated that these viruses are all members of a single closely related group, despite their rather wide phenotypic diversity. Fig. 1 upper left is a representative tryptic peptide map of an MuLV p30 from a BALB/c mouse. Twelve peptides have been numbered in Fig. 1 upper right to facilitate subsequent discussion. Some of the lighter spots (e.g., 1, 4, 6, and 9) always appear with a darker spot as a doublet. The spots that appear as doublets (1-2, 4-5, 6-7, 9-10, and 12) could represent the same tyrosine-containing peptide, the darker of the two containing monoiodotyrosine and the lighter one diiodotyrosine (26). As long as labeling and trypsin treatment conditions were held constant, identical peptide maps were generated from the same protein time after time. Several spots or "smudges" were seen in the right half of the MuLV p30 maps. These regions were usually not resolved as discrete spots but showed a similar pattern in most of the maps and were not considered in the comparisons described below.Fine Structural Differences in MuLV p3Os Are Excellent Markers for Specific Viral Strains. Although all MuLV p3Os obviously belong to a single structural group, there are minor differences in the peptide profiles indicative of small variances in the amino acid sequence of these molecules. These differences are reproducibly seen and are thus excellent markers for certain specific MULV strains.