Targeting of a heterodimeric membrane protein complex to the Golgi: rubella virus E2 glycoprotein contains a transmembrane Golgi retention signal.

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Abstract
Rubella virus (RV) envelope glycoproteins, E2 and E1, form a heterodimeric complex that is targeted to medial/trans-Golgi cisternae. To identify the Golgi targeting signal(s) for the E2/E1 spike complex, we constructed chimeric proteins consisting of domains from RV glycoproteins and vesicular stomatitis virus (VSV) G protein. The location of the chimeric proteins in stably transfected Chinese hamster ovary cells was determined by immunofluorescence, immunoelectron microscopy, and by the extent of processing of their N-linked glycans. A trans-dominant Golgi retention signal was identified within the C-terminal region of E2. When the transmembrane (TM) and cytoplasmic (CT) domains of VSV G were replaced with those of RV E2, the hybrid protein (G-E2TMCT+) was retained in the Golgi. Transport of G-E2TMCT+ to the Golgi was rapid (t1/2 = 10-20 min). The G-E2TMCT+ protein was determined to be distal to or within the medial Golgi based on acquisition of endo H resistance but proximal to the trans-Golgi network since it lacked sialic acid. Deletion analysis revealed that only the TM domain of E2 was required for Golgi targeting. Although the cytoplasmic domain of E2 was not necessary for Golgi retention, it was required for efficient transport of VSV G-RV chimeras out of the endoplasmic reticulum. When assayed in sucrose velocity sedimentations gradients, the Golgi-retained G-E2TMCT+ protein behaved as a dimer. Unlike virtually all other Golgi targeting signals, the E2 TM domain does not contain any polar amino acids. The TM and CT domains of E1 were not required for targeting of E2 and E1 to the Golgi indicating that a heterodimer of two integral membrane proteins can be retained in the Golgi by a single retention signal.