The V protein of Sendai virus (SeV) is nonessential to virus replication in cell culture but indispensable to viral pathogenicity in mice. The highly conserved cysteine-rich zinc finger-like domain in its carboxyl terminus is believed to be responsible for this viral pathogenicity. In the present study, we showed that the cysteine-rich domain of the SeV V protein could actually bind zinc by using glutathione-S-transferase fusion proteins. When the seven conserved cysteine residues at positions 337, 341, 353, 355, 358, 362, and 365 were replaced individually, the zinc-binding capacities of the mutant proteins were greatly impaired, ranging from 22 to 68% of that of the wild type. We then recovered two mutant SeVs from cDNA, which have V-C 341 S and V-C 365 R mutations and represent maximal and minimal zinc-binding capacities among the corresponding mutant fusion proteins, respectively. The mutant viruses showed viral protein synthesis and growth patterns similar to those of wild-type SeV in cultured cells. However, the mutant viruses were strongly attenuated in mice in a way similar to that of SeV V ⌬C , which has a truncated V protein lacking the cysteine-rich domain, by exhibiting earlier viral clearance from the mouse lung and less virulence to mice. We therefore conclude that the zinc-binding capacity of the V protein is involved in viral pathogenesis.Sendai virus (SeV), which belongs to the genus Respirovirus in the family Paramyxoviridae, is a respiratory tract pathogen of rodents. Like other members within the order Mononegavirales, it is an enveloped virus with a single-stranded, negativesense RNA genome of approximately 15.4 kb. The arrangement of the SeV genome starts with a short 3Ј leader sequence, followed by six genes encoding the structural proteins, including N (nucleocapsid), P (phosphoprotein), M (matrix), F (fusion), HN (hemagglutinin-neuraminidase), and L (large) proteins and terminates with a 5Ј trailer sequence (19).Among the six genes, the P gene was found to be unique in that it encodes not a single but multiple proteins. The colinear transcript encodes the P protein as well as the C, CЈ, Y 1 , and Y 2 proteins; these four proteins are translated in a shifted frame by alternative translational starts. The P gene also directs synthesis of an accessory mRNA for the V protein by inserting a pseudotemplated G residue at the specific editing site (30, 31). Consequently, the V and P proteins have the same 317 residues at the amino terminus (the P/V common region), while the V protein contains a 67-residue unique carboxyl terminus (the Vu region). This Vu region features a motif formed by seven cysteine residues (CX 3 CX 11 CXCX 2 CX 3 CX 2 C, where X refers to any amino acid residue), which are highly conserved in almost all the members of the subfamily Paramyxovirinae. The number and spacing of these cysteine residues resemble those of zinc finger structures and metalloproteins, and the V proteins of other members of the subfamily Paramyxovirinae, including simian virus 5 (SV5), measles virus, ...