Lysosomal sorting mutants of coronavirus E1 protein, a Golgi membrane protein

As a model for the intracellular sorting of Golgi membrane proteins, we are studying the E1 protein of the coronavirus Mouse Hepatitis Virus A59. The wild-type protein, when expressed from synthetic RNA, is localised in the Golgi complex. When the second and third of the three predicted membrane-spanning sequences were deleted from the protein, the resulting mutant was retained in the endoplasmic reticulum. In contrast, removal of the first and second membrane-spanning sequences allowed the protein to pass through the Golgi complex and reach the lysosomes. Likewise, when 40 amino acids were deleted from the C-terminal cytoplasmic part of E1, the truncated protein was transported to the lysosomes. We discuss the implications of these results for the structure of the E1 protein and the mechanism by which it is localised in the cell.