Human cytomegalovirus UL37 immediate-early regulatory proteins traffic through the secretory apparatus and to mitochondria.

The human cytomegalovirus (HCMV) UL36-38 immediate-early (IE) locus encodes the UL37 exon 1 (pUL37x1) and UL37 (gpUL37) regulatory proteins, which have anti-apoptotic activities. pUL37x1 shares its entire sequence, including a hydrophobic leader and an acidic domain, with the exception of one residue, with the amino terminus of gpUL37. gpUL37 has, in addition, unique N-linked glycosylation, transmembrane and cytosolic domains. A rabbit polyvalent antiserum was generated against residues 27-40 in the shared amino-terminal domain and a mouse polyvalent antiserum was generated against the full-length protein to study trafficking of individual UL37 proteins in human cells that transiently expressed gpUL37 or pUL37x1. Co-localization studies by confocal laser scanning microscopy detected trafficking of gpUL37 and pUL37x1 from the endoplasmic reticulum to the Golgi apparatus in permissive U373 cells and in human diploid fibroblasts (HFF). Trafficking of gpUL37 to the cellular plasma membrane was detected in unfixed HFF cells. FLAG-tagged gpUL37 trafficked similarly through the secretory apparatus to the plasma membrane. By using confocal microscopy and immunoblotting of fractionated cells, gpUL37 and pUL37x1 were found to co-localize with mitochondria in human cells. This unconventional dual trafficking pattern through the secretory apparatus and to mitochondria is novel for herpesvirus IE regulatory proteins.