THE HUMAN CYTOMEGALOVIRUS UL37 IMMEDIATE-EARLY REGULATORY PROTEIN IS AN INTEGRAL MEMBRANE N-GLYCOPROTEIN WHICH TRAFFICS THROUGH THE ENDOPLASMIC-RETICULUM AND GOLGI-APPARATUS

The human cytomegalovirus (HCMV) UL37 immediate-early gene is predicte d to encode a type I membrane-bound glycoprotein, gpUL37. Following ex pression of the UL37 open reading frame in vitro, its signals for tran slocation and N-glycosylation were recognized by microsomal enzymes. I ts orientation in the microsomes is that of a type I protein. gpUL37 p roduced in HCMV-infected human cells was selectively immunoprecipitate d by rabbit polyvalent antiserum generated against the predicted uniqu e domains of the UL37 open reading frame and migrated as an 83- to 85- kDa protein. Tunicamycin treatment, which inhibits N-glycosylation, in creased the rate of migration of the UL37 protein to 68 kDa, verifying its modification by N-glycosylation in HCMV-infected cells. Consisten t with this observation, gpUL37 was found to be resistant to digestion with either endoglycosidase F or H but sensitive to peptide N-glycosi dase F digestion. These results suggested that gpUL37 is N-glycosylate d and processed in both the endoplasmic reticulum (ER) and the Golgi a pparatus. Direct demonstration of passage of gpUL37 through the ER and the Golgi was obtained by confocal microscopy. gpUL37 colocalized wit h protein disulfide isomerase, a protein resident in the ER, and with a Golgi protein. Subcellular fractionation of HCMV-infected cells demo nstrated that gpUL37 is an integral membrane protein. Taken together, our results demonstrate that the HCMV gpUL37 immediate-early regulator y protein is a type I integral membrane N-glycoprotein which traffics through the ER and the Golgi network.