The Adenovirus E3-6.7K Protein Adopts Diverse Membrane Topologies following Posttranslational Translocation

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Abstract
The E3 region of adenovirus codes for several membrane proteins, most of which are involved in immune evasion and prevention of host cell apoptosis. We explored the topology and targeting mechanisms of E3-6.7K, the most recently described member of this group, by using an in vitro translation system supplemented with microsomes. Here, we present evidence that E3-6.7K, one of the smallest signal-anchor proteins known, translocates across the membrane of the endoplasmic reticulum in a posttranslational, ribosome-independent, yet ATP-dependent manner, reminiscent of the translocation of tail-anchored proteins. Our analysis also demonstrated that E3-6.7K could achieve several distinct topological fates. In addition to the previously postulated type III orientation (N-luminal/C-cytoplasmic, termed Ntm E3-6.7K), we detected a tail-anchored form adopting the opposite orientation (N-cytoplasmic/C-luminal, termed Ctm E3-6.7K) as well as the possibility of a fully translocated form (N and C termini are both translocated, termed NC E3-6.7K). Due to the translocation of a positively charged domain, both the Ctm E3-6.7K and NC E3-6.7K topologies of E3-6.7K constitute exceptions to the “positive inside” rule. The Ntm E3-6.7K and NC E3-6.7K are the first examples of posttranslationally translocated proteins in higher eukaryotes that are not tail anchored. Distinct topological forms were also found in transfected cells, as both N and C termini of E3-6.7K were detected on the extracellular surface of transfected cells. The demonstration of unexpected topological forms and translocation mechanisms for E3-6.7K defies conventional thinking about membrane protein topogenesis and advises that both the mode of targeting and topology of signal-anchor proteins should be determined experimentally.