The membrane-spanning segment of invariant chain (Iγ) contains a potentially cleavable signal sequence

The human invariant chain (Iγ) of class II histocompatibility antigens spans the membrane of the endoplasmic reticulum once. It exposes a small amino-terminal domain on the cytoplasmic side and a carboxyterminal, glycosylated domain on the exoplasmic side of the membrane. When the exoplasmic domain of Iγ is replaced by the cytoplasmic protein chloramphenicol acetyltransferase (CAT), CAT becomes the exoplasmic, glycosylated domain of the resulting membrane protein IγCAT∗. Deletion of the hydrophilic cytoplasmic domain from IγCAT gives rise to a secreted protein from which an amino-terminal segment is cleaved, most likely by signal peptidase. We conclude that the membrane-spanning region of Iγ contains a signal sequence in its amino-terminal half and that hydrophilic residues at the amino-terminal end of a signal sequence can determine cleavage by signal peptidase.