Transmembrane domain length affects charge-mediated retention and degradation of proteins within the endoplasmic reticulum.

Previous studies have shown that the presence of potentially charged amino acid residues within the transmembrane domains of type I integral membrane proteins can result in protein retention and, in some cases, degradation within the endoplasmic reticulum (ER). An apparent exception to this observation is the CD3-epsilon chain of the T-cell antigen receptor complex, which is relatively stable in spite of having a transmembrane aspartic acid residue. A chimeric protein (T epsilon T) made by replacing the transmembrane domain of the Tac antigen with that of CD3-epsilon was normally transported to the cell surface, indicating that the transmembrane domain of CD3-epsilon was essentially unable to confer the phenotype of ER retention and degradation to another protein. Progressive shortening of the T epsilon T transmembrane domain, however, resulted in increasing retention and degradation of the mutant proteins in the ER. Conversely, a mutant Tac protein containing a single aspartic acid residue in its transmembrane domain was found to be retained and degraded in the ER, but when the transmembrane domain was lengthened, ER retention and degradation of the protein were abrogated. The aspartic acid residue in the transmembrane domain of all of these mutant proteins could mediate assembly with another protein having an arginine residue in its transmembrane domain, independent of the length of the transmembrane sequence. These findings demonstrate that the length of the hydrophobic transmembrane sequence has a critical influence on the ability of potentially charged transmembrane residues to cause protein retention and degradation in the ER.