Carboxyl-terminal targeting and novel post-translational processing of JAW1, a lymphoid protein of the endoplasmic reticulum

Jaw1 is a lymphoid-restricted protein localized to the cytoplasmic face of the endoplasmic reticulum (ER) and is a member of a recently recognized class of integral membrane proteins that contain carboxyl-terminal membrane anchors. The carboxyl-terminal 71 amino acids of the Jaw1 protein, which contain a hydrophobic membrane spanning region, are sufficient to target a heterologous protein to the ER. By discontinuous sucrose gradient ultracentrifugation, differential sedimentation was noted for the four major Jaw1 protein isoforms, with two of the forms predominantly soluble and two microsome-bound. Pulse-chase immunoprecipitations suggest a post-translational modification of two major isoforms of the protein resulting in an increase in mobility on SDS-polyacrylamide gel electrophoresis. In vitro translation studies are compatible with a post-translational processing event that results in cleavage of a short 36 amino acid lumenal domain. These findings define a carboxyl-terminal domain of the Jaw1 protein that is both necessary and sufficient for ER localization. In addition, the processing of the small lumenal domain of Jaw1 represents a novel post-translational protein modification performed by the endoplasmic reticulum.