Minor capsid proteins of simian virus 40 are dispensable for nucleocapsid assembly and cell entry but are required for nuclear entry of the viral genome.
We investigated the roles of simian virus 40 capsid proteins in the viral life cycle by analyzing point mutants in Vp1 and Vp2/3, as well as a deletion mutant lacking the Vp2/3 coding sequence. The Vp1 mutants (V243E and L245E) and the Vp2/3 mutants (F157E-I158E and P164R-G165E-G166R) were previously shown to be defective in Vp1-Vp2/3 interaction and to be noninfectious or poorly infectious, respectively. Here, we show that all these point mutants form stable particles following DNA transfection into cells. The Vp2/3-mutant particles contained very low levels of Vp2/3, whereas the Vp1 mutant particles contained no detectable Vp2/3. As expected, the deletion mutant also formed particles that were noninfectious. We further characterized the two Vp1 point mutants and the deletion mutant. All three mutant particles comprised Vp1 and histone-associated viral DNA, and all were able to enter cells. However, the mutant complexes failed to associate with host importins (owing to the loss of the Vp2/3 nuclear localization signal), and the mutant viral DNAs prematurely dissociated from the Vp1s, suggesting that the nucleocapsids did not enter the nucleus. Consistently, all three mutant particles failed to express large T antigen. Together, our results demonstrate unequivocally that Vp2/3 is dispensable for the formation of nucleocapsids. Further, the nucleocapsids' ability to enter cells implies that Vp1 contains the major determinants for cell attachment and entry. We propose that the major role of Vp2/3 in infectivity is to mediate the nuclear entry of viral DNA.
Nakanishi
Akira
A
Molecular Biology Institute and Department of Molecular, Cell, and Developmental Biology, 456 Boyer Hall, University of California, Los Angeles, 611 East Charles E. Young Dr., Box 951570, Los Angeles, CA 90095-1570, USA.
Itoh
Noriko
N
Li
Peggy P
PP
Handa
Hiroshi
H
Liddington
Robert C
RC
Kasamatsu
Harumi
H
D019943:Amino Acid Substitution; D000818:Animals; D001692:Biological Transport; D036022:Capsid Proteins / Q000235:genetics / Q000502:physiology*; D002460:Cell Line; D002467:Cell Nucleus / Q000821:virology; D004279:DNA, Viral / Q000378:metabolism*; D016679:Genome, Viral; D000882:Haplorhini; D028884:Karyopherins / Q000378:metabolism; D019251:Nucleocapsid / Q000378:metabolism*; D017354:Point Mutation; D011485:Protein Binding; D017384:Sequence Deletion; D013539:Simian virus 40 / Q000235:genetics / Q000502:physiology*; D019065:Virus Assembly; D053586:Virus Internalization
D036022:Capsid Proteins; D004279:DNA, Viral; D028884:Karyopherins; C062481:VP1 protein, polyomavirus; C085902:VP3 protein, Simian virus 40; C514199:Vp2 protein, simian virus 40