A structural rationale for SV40 Vp1 temperature-sensitive mutants and their complementation

Viral calciomics: interplays between Ca2+ and virus

Ca²⁺ is one of the most universal and versatile signaling molecules and is involved in almost every aspect of cellular processes. Viruses are adept at utilizing the universal Ca²⁺ signal to create a tailored cellular environment that meets their own demands. This review summarizes most of the known mechanisms by which viruses perturb Ca²⁺ homeostasis and utilize Ca²⁺ and cellular Ca²⁺-binding proteins to their benefit in their replication cycles. Ca²⁺ plays important roles in virion structure formation, virus entry, viral gene expression, posttranslational processing of viral proteins and virion maturation and release. As part of the review, we introduce an algorithm to identify linear “EF-hand” Ca²⁺-binding motifs which resulted in the prediction of a total of 93 previously unrecognized Ca²⁺-binding motifs in virus proteins. Many of these proteins are nonstructural proteins, a class of proteins among which Ca²⁺ interactions had not been formerly appreciated. The presence of linear Ca²⁺-binding motifs in viral proteins enlarges the spectrum of Ca²⁺–virus interplay and expands the total scenario of viral calciomics.

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.

Identification of amino acid residues within simian virus 40 capsid proteins Vp1, Vp2, and Vp3 that are required for their interaction and for viral infection

Interaction of simian virus 40 (SV40) major capsid protein Vp1 with the minor capsid proteins Vp2 and Vp3 is an integral aspect of the SV40 architecture. Two Vp3 sequence elements mediate Vp1 pentamer binding in vitro, Vp3 residues 155 to 190, or D1, and Vp3 residues 222 to 234, or D2. Of the two, D1 but not D2 was necessary and sufficient to direct the interaction with Vp1 in vivo. Rational mutagenesis of Vp3 residues (Phe157, Ile158, Pro164, Gly165, Gly166, Leu177, and Leu181) or Vp1 residues (Val243 and Leu245), based on a structural model of the SV40 Vp1 pentamer complexed with Vp3 D1, was carried out to disrupt the interaction between Vp1 and Vp3 and to study the consequences of these mutations for viral viability. Altering these residues to bulky, charged residues blocked the interaction in vitro. When these alterations were introduced into the viral genome, they reduced viral viability. Mutants with alterations in Vp1 Val243, Leu245, or both to glutamate were nearly nonviable, whereas those with Vp3 alterations reduced, but did not eliminate, viability. Our results defined the residues of Vp1 and the minor capsid proteins that are essential for both the interaction of the capsid proteins and viral viability in permissive cells.