The carboxyl terminus of the human cytomegalovirus UL37 immediate-early glycoprotein is conserved in primary strains and is important for transactivation

The human cytomegalovirus (HCMV) UL37 exon 3 (UL37x3) open reading frame (O RF) encodes the carboxyl termini of two immediate-early glycoproteins (gpUL 37 and gpUL37(M)). UL37x3 homologous sequences are not required for mouse c ytomegalovirus (MCMV) growth in vitro; yet, they are important for MCMV gro wth and pathogenesis in vivo. Similarly, UL37x3 sequences are dispensable f or HCMV growth in culture, but their requirement for HCMV growth in vivo is not known. To determine this requirement, we directly sequenced the comple te UL37x3 gene in multiple HCMV primary strains. A total of 63 of the 310 a mino acids in the UL37x3 ORF differ nonconservatively in one or more HCMV p rimary strains, The HCMV UL37x3 genetic diversity is nonrandom:the N-glycos ylation (46/186 aa) and basic (9/15 aa) domains have the highest proportion of non-conservative variant amino acids. Nonetheless, most (15/17 signals) of the N-glycosylation signals are retained in all HCMV primary strains. M oreover, new N-glycosylation signals are encoded by 5/20 primary strains. I n sharp contrast, the UL37x3 transmembrane (TM) ORF completely lacks divers ity in all 20 HCMV sequenced primary strains, and only 1 of 28 cytosolic ta il residues differs non-conservatively. To test the functional significance of the conserved carboxyl terminus, gpUL37 mutants lacking the TM and/or c ytosolic tail were tested for transactivating activity. The gpUL37 carboxyl -terminal mutants are partially defective in hsp70 promoter transactivation even though they trafficked similarly to the wild-type protein into the en doplasmic reticulum and to mitochondria. From these results, we conclude th at N-glycosylated gpUL37, particularly its TM and cytosolic domains, is imp ortant for HCMV growth in humans.