The transmembrane domain of the severe acute respiratory syndrome coronavirus ORF7b protein is necessary and sufficient for its retention in the Golgi complex

The severe acute respiratory syndrome coronavirus (SARS-CoV) ORF7b (also called 7b) protein is an integral membrane protein that is translated from a bicistronic open reading frame encoded within subgenomic RNA 7. When expressed independently or during virus infection, ORF7b accumulates in the Golgi compartment, colocalizing with both cis- and trans-Golgi markers. To identify the domains of this protein that are responsible for Golgi localization, we have generated a set of mutant proteins and analyzed their subcellular localizations by indirect immunofluorescence confocal microscopy. The N- and C-terminal sequences are dispensable, but the ORF7b transmembrane domain (TMD) is essential for Golgi compartment localization. When the TMD of human CD4 was replaced with the ORF7b TMD, the resulting chimeric protein localized to the Golgi complex. Scanning alanine mutagenesis identified two regions in the carboxy-terminal portion of the TMD that eliminated the Golgi complex localization of the chimeric CD4 proteins or ORF7b protein. Collectively, these data demonstrate that the Golgi complex retention signal of the ORF7b protein resides solely within the TMD.