(Na+ + K+)-ATPase in C6 glioma and rat cerebrum

The content and distribution of the membranee-bound enzyme (Na+ + K+)-ATPase in a rat cerebral C6 glioma was determined by immunocytochemistry, immunoblots and enzyme assay. In the C6 glioma cell culture (Na+ + K+)-ATPase activity was about 20% of (Mg2+ + Na+ + K+)-ATPase activity. However, (Mg2+ + Na+ + K+)-ATPase activity in the cerebral C6 gliomas was very close to Mg2+ baseline and not significantly increased by Na+ and K+. As shown by immunobloting, (Na+ + K+)-ATPase catalytic subunit was detected in excised samples of control cerebrum and as a trace in the intracerebral portions of C6 glioma but not at all in the extracranial portions of C6 glioma or in C6 glioma cell culture. (Na+ + K+)-ATPase was not detected immunocytochemically in paraffin sections of the extracranial or intracerebral portions of rat cerebral C6 glioma. The absence of staining for (Na+ + K+)-ATPase clearly demarcated projections of glioma within normal brain. These results suggest that C6 glioma has little if any expression of (Na+ + K+)-ATPase in vitro or in vivo. The small amount of enzyme epitope in the intracerebral portions represents contamination by normal cerebrum in the extracts.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/26689/1/0000236.pd

Polyproteins related to the major core protein of mouse mammary tumor virus.

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