The molecular basis of cytomegalovirus glycoprotein complex II and its role in disease pathogenesis

Abstract

Human cytomegalovirus causes life-threatening disease in immunocompromised patients and congenitally infected newborns. The virus envelope carries key glycoproteins which are crucial for viral replication, entry, and spread. Some of these glycoproteins have polymorphic features resulting in distinct genotypes that suggested to have an association with disease severity. However, the glycoprotein complex II, particularly gM, is poorly understood in this regard although it is important for viral replication. Glycoprotein M is known to be highly conserved and has not been considered to exist as variable genotypes among HCMV strains. The overall aim of this study was to investigate the role of glycoprotein M in complex II of HCMV. This study confirms the conserved nature of gM but identified three distinct genotypes of it, the differences between the three genotypes are small. Analysis of amino acid sequences of gM clustered gM1 and gM2 together while gM3 separated in an independent group. Analysis of the hydrophobicity of the predicted amino acid sequences, of the transmembrane topology of the predicted amino acid sequences, and predictive modelling of their 3D structure showed that there was 97 to 99% similarity of genotypes. An unexpected finding was the importance of change in the C-terminal tail of the protein predicted to be present on the inside of the virion envelope embedded within the tegument. Here as little as one amino acid difference was sufficient to differentiate gM3 from the other genotypes a finding reinforced by in vitro experiments with a mutated form of HCMV Merlin in which the HCMV Merlin form of M was substituted with a gM1 sequence derived from HCMV AD169. The high degree of conservation of gM may reflect that in nature gM must form a covalent complex with gN in the HCMV surface glycoprotein complex II. If the complex cannot form, then virus viability is impacted. However, studies of mutated HCMV Merlin suggest that gM3 has not only physical variabilities (including glycosylation) from the other genotypes but may be able to better modulate the immune response than viruses having the alternate genotype. In sequences downloaded from Genbank, 53% belonged to gM genotype 3, however since all were clinical isolates, the data cannot represent the overall types of gM circulating in the community. A larger population-based study would be required to point out if gM3 that suggested to be associated with higher disease severity based on indicators of this work. Furthermore, the relationship between gM and the high polymorphic gN, which forms a complex with gM, should be considered in further study. In fibroblasts and in SH-SY5Y neuroblastoma cells, gM3 type produced higher viruses in SH-SY5Y cells suggesting higher virulence. In vitro study of immediate innate immune response to HCMV infection of Merlin mutant strains, gM3 type had higher immunomodulatory activity compared to other two types, however these results require a verification in vivo. Also, future work should consider other glycoproteins genotypes, beside gM, in a search for potential biomarkers of predicting disease outcomes.