A beta-stranded motif drives capsid protein oligomers of the parvovirus minute virus of mice into the nucleus for viral assembly

The determinants of nuclear import in the VP-1 and VP-2 capsid proteins of the parvovirus minute virus of mice strain i (MVMi) synthesized in human fi broblasts were sought by genetic analysis in an infectious plasmid, Immunof luorescence of transfected cells revealed that the two proteins were involv ed in cooperative cytoplasmic interactions for nuclear cotransport, However , while VP-1 translocated regardless of extension of deletions and did not form capsid epitopes by itself, VP-2 seemed to require cytoplasmic folding and the overall conformation for nuclear transport. The sequence (528)KGKLT MRAKLR(538) was found necessary for nuclear uptake of VP-2, even though it was not sufficient to confer a nuclear localization capacity on a heterolog ous protein, In the icosahaedral MVMi capsid, this sequence forms the carbo xy end of the amphipathic beta-strand I (beta I), and all its basic residue s are contiguously positioned at the face that in the unassembled subunit w ould be exposed to solvent. Mutations in singly expressed VP-2 that either decrease the net basic charge of the exposed face (K530N-R534T), perturb th e hydrophobicity of the opposite face (L531E), or distort the beta I confor mation (G529P) produced cytoplasmic subviral oligomers, Particle formation by beta I mutants indicated that the basic residues clustered at one face o f beta I drive VP oligomers into the nucleus preceding and uncoupled to ass embly and that the nuclear environment is required for MVMi capsid formatio n in the infected cell, The degree of VP-1/VP-2 transport cooperativity sug gests that VP trimers are the morphogenetic intermediates translocating thr ough the nuclear pore. The results support a model in which nuclear transpo rt signaling preserves the VP-1/VP-2 stoichiometry necessary for efficient intranuclear assembly and in which the beta-stranded VP-2 nuclear localizat ion motif contributes to the quality control of viral morphogenesis.