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Polyproteins related to the major core protein of mouse mammary tumor virus
Abstract:
The mouse mammary tumor virus (MuMTV) contains several low-molecular-weight proteins which, together with the genomic RNA, constitute the core structure of the virion. The most abundant protein in the core is the 27,000-dalton protein (p27), and, by analogy to the type C viruses, this protein probably forms the core shell. In mouse mammary tumor cell lines (GR and Mm5MT) producing MuMTV the major p57 antigenic specificity resides in a large protein, which migrates in polyacrylamide gels as a doublet of 77,000 and 75,000 daltons (p 77/75). A series of lower-molecular-weight proteins, p61, p48, p38, and p34, is also present in small amounts and is probably derived by proteolytic cleavage of the p 77/75. These proteins have been identified by immunoprecipitation with monospecific antiserum, and their sequence relatedness to p27 has been determined by an analysis of the peptides after trypsin digestion. After a 15-min pulse with [35S]-methionine, all of the p27-related proteins in these cell lines were labelled and, during a subsequent chase, progressively disappeared. The p27 was labeled poorly during the pulse, but the amount of label in this protein increased during the chase. A quantitation of these experiments suggested that the majority of the p27-related proteins were quite rapidly turned over in these cell lines. Hence, if p27 is derived by a progressive proteolytic cleavage mechanism, then the process is inefficient in the GR cells and only moderately efficient in the Mm5MT cells. When MuMTV was isolated from the culture medium of these cells harvested at 5-min intervals, the major p27-related protein was p34. The p27 accounted for only 29% of the anti-p27 serum immunoprecipitable proteins compared to 95% in virus isolated from an 18-h harvest. Incubation of the rapid-harvest virus at 37 degrees C for 2 h resulted in some conversion of p34 to p27. These results suggest that some of the p27 in MuMTV is formed in the virions by proteolytic cleavage of p34.
Insights
Mouse mammary tumor virus (MuMTV) core proteins, including p27, are processed through proteolytic cleavage. The p34 protein is identified as a precursor to p27, with cleavage occurring within the virion.
Area of Science:
- Virology
- Molecular Biology
- Biochemistry
Background:
- Mouse mammary tumor virus (MuMTV) possesses low-molecular-weight proteins forming its core structure.
- The 27,000-dalton protein (p27) is the most abundant core protein, likely forming the core shell.
- MuMTV-producing cell lines express a major p57 antigenic specificity in a large protein (p77/75) and related lower-molecular-weight proteins (p61, p48, p38, p34).
Purpose of the Study:
- To investigate the processing and origin of MuMTV core proteins, particularly p27.
- To determine the relationship between p77/75 and lower-molecular-weight MuMTV proteins.
- To elucidate the mechanism of p27 formation within MuMTV virions.
Main Methods:
- Pulse-chase experiments using [35S]-methionine labeling in MuMTV-producing cell lines (GR and Mm5MT).
- Immunoprecipitation with monospecific antiserum to identify and quantify MuMTV proteins.
- Peptide analysis following trypsin digestion to assess sequence relatedness.
- Isolation and incubation of MuMTV from culture medium harvested at different intervals.
Main Results:
- Pulse-chase experiments indicated rapid turnover of p27-related proteins, with p27 accumulating during the chase, suggesting it's a later product.
- In MuMTV isolated from early harvests, p34 was the predominant p27-related protein, while p27 comprised 95% in late harvests.
- Incubation of early-harvest MuMTV at 37°C led to the conversion of p34 to p27.
Conclusions:
- The p27-related proteins, including p34, are likely derived from proteolytic cleavage of a larger precursor.
- The p34 protein serves as a precursor to p27.
- A significant portion of p27 is formed by proteolytic cleavage of p34 within the MuMTV virion after its assembly.