The products of human cytomegalovirus genes UL23, UL24, UL43 and US22 are tegument components

Abstract

We have investigated the human cytomegalovirus (HCMV) US22 gene family members UL23, UL24, UL43 and US22. Specific antibodies were generated to identify pUL23 (33 kDa), pUL24 (40 kDa) and pUL43 (48 kDa), while pUS22 was identified by monoclonal antibody HWLF1. A C-terminally truncated UL43 product (pUL43t; 21 kDa) produced by a deletion mutant was also investigated. The UL24 and UL43 genes were expressed with early-late (gamma1) and true-late (gamma2) kinetics, respectively. Immunoblot and immuno-EM studies demonstrated that pUL23, pUL24, pUL43 and pUS22 were virion tegument components. Immunofluorescence and immuno-EM studies showed that pUL23, pUL24, pUL43 and pUL43t were located in cytoplasmic protein aggregates, manifesting two forms: complex juxtanuclear structures and smaller, membrane-bound aggregates resembling dense bodies. The complex-type aggregate is a putative site of particle maturation. Because pUL43t was present in protein aggregates, but under-represented in virus particles compared to pUL43, it was concluded that N-terminal sequences target pUL43 to protein aggregates and that C-terminal sequences are important for incorporation into particles. Since three other US22 family products (pUL36, pTRS1 and pIRS1) are documented tegument components, at least seven of the twelve US22 family genes encode tegument proteins, suggesting that the products of the remaining five genes might be similarly located. These findings demonstrate a common biological feature among most, if not all, US22 family proteins and implicate the family in events occurring immediately after virus penetration.