Charge microheterogeneity of the major capsid protein of polyoma virus

Abstract

The behavior in isoelectric focusing of the major capsid polypeptide VP1 of several strains of polyoma virus was studied. Two previously recognized phenomena were reexamined, namely, (i) the separation of the VP1 polypeptide into multiple subspecies differing only slightly from each other in apparent isoelectric point and (ii) strain differences in the overall apparent net charge of the family of VP1 subspecies. It was found that the pattern of subspecies was reproducible when focusing was initiated from either the basic or acidic region of the gel, keeping the ampholyte mixture constant. However, individual subspecies were unstable, and labeled polypeptide could be shifted dramatically by either refocusing of separated subspecies or by altering the concentration of ampholytes. These findings suggest that protein-protein and protein-ampholyte interactions play an important role in the generation of this charge heterogeneity. The basis for the overall charge difference between the VP1 of 3049 virus and several other strains (lpD, IpS, ts59, and A2) was studied, using recombinant viruses constructed of specific sequences derived from 3049 and lpD genomes. The portion of the VP1 polypeptide carrying the altered charge could be mapped to the body of the molecule 3' to the HindIII site at 45.0 map units (3,918 base pairs). This clearly segregates the VP1 charge phenotype from the cyc phenotype of 3049 in which capsid proteins are overproduced and accumulate in the cytoplasm of infected cells.

Comparison of nonphosphorylated and phosphorylated species of polyomavirus major capsid protein VP1 and identification of the major phosphorylation region

“…It may result from additional, as yet uncharacterized, posttranslational modifications such as acetylation, carboxylation or decarboxylation, sulfation, methylation, etc. Alternatively, it has been suggested that some of the heterogeneity may result fom conformational differences, protein-protein interactions, or ampholyte binding (20). We are continuing to investigate this important question.…”

Chemical cleavage of polyomavirus major structural protein VP1: identification of cleavage products and evidence that the receptor moiety resides in the carboxy-terminal region

“…Isoelectric focusing of disrupted polyomavirus virions resolves at least six isospecies, which have been designated A through F (3,19,20). Earlier work performed in our laboratory suggested that the phosphorylated species D, E, and F, which together comprise only about 12 to 15% of the total VP1 present in polyomavirus virions, perform the functions of viral hemagglutinin and receptor (3).…”