A Golgi retention signal in a membrane-spanning domain of coronavirus E1 protein

Contrasting results with an El protein from a different coronavirus, mouse hepatitis virus, have recently been reported by Armstrong et al . (1) . They found that an El protein lacking the carboxy-terminal 40 amino acids, as well as a deletion mutant comparable to our Om1,2 was not retained in the Golgi region . (Although these proteins were not retained in the Golgi region, they were detected in lysosomes, Figure 7 . Indirect immunofluorescence microscopy of Om2,3 with mutations in the ml domain . Transfected COS cells were stained as in Fig . 6 . When ml was the only membrane-spanning domain in the protein, all of the Ile mutations resulted in transport of the mutant proteins to the plasma membrane. Mutation of Leu30 to Gln (dm2,3/LQjo) had no effect on Golgi localization . Bar, 10 ,.m. not at the plasma membrane) . We have produced a number of deletions in the cytoplasmic tail of IBV El, and have seen no effect on Golgi localization (unpublished results) . Although localization of the mouse hepatitis virus El protein in transfected cells at the electron microscopic level has not been reported, the protein appears to acquire carbohydrate modifications characteristic of the late Golgi region when Swift and Machamer Golgi Retention Signal expressed in COS cells (39) . Thus, it is possible that the mouse hepatitis virus El protein reaches a later Golgi subcompartment than the IBV El protein, and that the retention mechanism could be different . Interestingly, both TGN38 (a trans-Golgi network protein ; reference 25) and Kex2p (believed to be a late Golgi protease in S. cerevisiae ; reference 11) are not retained in the Golgi complex when their cyto- Fluorograms were quantitated by densitometry, and the amount of processed (polylactosaminoglycan containing) oligosaccharides was determined by subtracting the amount of unprocessed material (endo H sensitive) from the total (N-glycanase sensitive) . The percentage of El with processed oligosaccharides is shown below each set of three lanes .