Subcellular trafficking and functional importance of herpes simplex virus type 1 glycoprotein M domains

Herpes simplex virus type 1 (HSV-1) glycoprotein M (gM/UL10) is a 473 aa type III transmembrane protein that resides in various membrane compartments. HSV-1 gM contains' several putative trafficking motifs, but their functional relevance remains to be elucidated. We show here that transiently expressed gM 19-343 was sufficient for transport to the trans-Golgi network (TGN), whilst gM 133-473, where the first two transmembrane domains were deleted, and gM 1-342, which lacked the final residue of the last transmembrane domain, were retained in the endoplasmic reticulum (ER), indicating that all transmembrane domains are required for proper folding and ER exit. A series of bacterial artificial chromosome mutants revealed that in addition to the authentic start codon, translation of gM can be initiated at methionine 19 and 133/135. Whilst a protein lacking the first 18 residues supported WT-like growth, gM 133/135-473 resulted in reduced plaque diameters resembling a UL10 deletion mutant. An HSV-1 mutant encoding gM 1-342 showed similar growth characteristics and accumulated non-enveloped cytoplasmic particles, whilst gM 1-343 resulted in a gain of function, indicating that all transmembrane domains of the protein are important for viral growth. A C-terminal extension further supported viral propagation; however, the C-terminal trafficking motifs (residues 423-473) were completely dispensable. We propose a functional core within gM 19-343 comprised of all transmembrane domains that is sufficient to target the protein to the TGN, a favoured site for envelopment, and to support viral functions.