ABSTRACT. A major capsid protein (MCP) gene homologue of porcine cytomegalovirus (PCMV) was identified. Sequence analysis indicated that the PCMV MCP gene is 4,026 nucleotides in length encoding a protein of 1,341 amino acid residues. The predicted molecular weight of the PCMV MCP is 151,456 Da, equivalent to those of other herpesvirus MCP counterparts. Phylogenetic analysis using herpesviral MCP gene sequences confirmed that PCMV is a betaherpesvirus with higher homology with human herpesvirus-6 and -7 than human and mouse cytomegaloviruses. The serum of pig experimentally infected with PCMV did not react with bacterially expressed MCP, suggesting that the PCMV MCP may not be related to the humoral immune response in the course of PCMV infection. Also, we established polymerase chain reaction (PCR) protocols using primers corresponding to MCP gene sequences for detection of PCMV infection. The PCR protocol would be effective for the diagnosis of slow-growing PCMV infection, for which traditional methods involving virus-isolation are not useful. KEY WORDS: betaherpesvirus, major capsid protein, polymerase chain reaction, porcine cytomegalovirus.J. Vet. Med. Sci. 63(6): 609-618, 2001 Porcine cyomegalovirus (PCMV), first described in 1955 [6], usually induces a silent infection in adult pigs but often a fatal, generalized infection in piglets. In utero infection in sows can cross the placenta and infect the fetuses leading to fetal death or birth of weak piglets [3,17]. As PCMV exhibits a relatively protracted replication cycle, a slowly developing cytopathology characterized by cytomegaly, and a restricted host range, it is grouped into the subfamily Betaherpesvirinae [18]. Recently, we and other research groups confirmed that PCMV is a betaherpesvirus based on sequence analyses of DNA polymerase gene fragments [19,29].The major capsid protein (MCP) gene appears to be conserved among the herpesviruses [5,15], suggesting that MCP should play essential roles in vital processes in the viral replication cycle such as assembly and maturation. Herpes simplex virus-1 (HSV-1) has been shown to encode seven capsid proteins. Of these, MCP is the structural subunit of the hexons and pentons, and is the principal structural element making up the matrix of the icosahedral capsid [16]. In addition, in some herpesviruses such as HSV-1 and varicella-zoster virus (VZV), MCPs serve as an antigen for the humoral immune response during the course of infection, being highly reactive with human antisera as well as being responsible for cell-mediated immunity [7,20,27].The MCPs of many herpesviruses have been shown to have similar molecular masses, e.g., between 135 kDa and 160 kDa for human herpesviruses [2,4,10,14,15]. In this study, we identified the MCP gene of PCMV and expressed the protein in a bacterial expression system. In addition, we also analyzed the evolutionary position of PCMV in the herpesvirus group using MCP sequence information. We also established a polymerase chain reaction (PCR) protocol for specific detection ...