The M25 gene products are critical for the cytopathic effect of mouse cytomegalovirus

Abstract

Cell rounding is a hallmark of the cytopathic effect induced by cytomegaloviruses. By screening a panel of deletion mutants of mouse cytomegalovirus (MCMV) a mutant was identified that did not elicit cell rounding and lacked the ability to form typical plaques. Altered cell morphology was assigned to the viral M25 gene. We detected an early 2.8 kb M25 mRNA directing the synthesis of a 105 kDa M25 protein, and confirmed that a late 3.1 kb mRNA encodes a 130 kDa M25 tegument protein. Virions lacking the M25 tegument protein were of smaller size because the tegument layer between capsid and viral envelope was reduced. The ΔM25 mutant did not provoke the rearrangement of the actin cytoskeleton observed after wild-type MCMV infection, and isolated expression of the M25 proteins led to cell size reduction, confirming that they contribute to the morphological changes. Yields of progeny virus and cell-to-cell spread of the ΔM25 mutant in vitro were diminished and replication in vivo was impaired. The identification of an MCMV gene involved in cell rounding provides the basis for investigating the role of this cytopathic effect in CMV pathogenesis.

Cytomegalovirus Generates Assembly Compartment in the Early Phase of Infection by Perturbation of Host-Cell Factors Recruitment at the Early Endosome/Endosomal Recycling Compartment/Trans-Golgi Interface

“…As expected, M25 was expressed in the E phase in approx. 60% of cells (Figure 2A), displaying a nuclear punctate pattern (Kutle et al, 2017), and in the L phase (24 and 48 hpi) in approx. 3/4 of cells.…”

“…(B) Organization of the MCMV life cycle and expression kinetics of MCMV genes that encode proteins of interest for this study. The schematic presentation is based on the published data (Scrivano et al, 2010;Marcinowski et al, 2012;Kutle et al, 2017). IE, immediate early phase; E, early phase; L, late phase; 11/2-column fitting image.…”

“…M55 and M74 build MCMV envelope glycoprotein complexes and are expected at membranous organelles of the secretory pathway (Scrivano et al, 2010). M25 is expressed in two main forms: 105 kDa M25 that is expressed in the nucleus during the E and L phase of infection, and 130 kDa M25 that is expressed only in the L phase of infection in both the nucleus and cytoplasm (Kutle et al, 2017), and incorporates into virions as a dominant tegument protein (Kattenhorn et al, 2004;Kutle et al, 2017).…”

The Abundant Tegument Protein pUL25 of Human Cytomegalovirus Prevents Proteasomal Degradation of pUL26 and Supports Its Suppression of ISGylation

“…M25 can be found in both the nucleus and the cytoplasm, whereas pUL25 is strictly cytoplasmic (reference 24 and data not shown). Most striking is that deletion of M25 alters the cellular cytoskeleton, leading to an abrogation of MCMV-induced cell rounding, and the virion displays a reduced size (41). Neither feature was seen in our analyses of pUL25.…”

“…pUL25 orthologous proteins can be found in other members of the Betaherpesvirus family (41,42). The murine orthologue M25 has recently been investigated (41). That protein displays features that distinguish it from pUL25.…”