Transport of an immunoglobulin light chain fragment across the endoplasmic reticulum does not require an amino terminal variable region: Implications for the signal hypothesis

The co-translational processing of the kappa light chain constant region fragment was investigated in a Krebs ascites cell-free system. The fragment which is devoid of the entire variable region was initially synthesized as a precursor which contains a signal (pre) peptide. Addition of microsomal membranes to the reaction mixtures resulted in the synthesis of a 11,600 dalton product which was sequestered in the vesicles. Amino acid sequence analysis indicated that the pre-peptide had been removed. The data show that despite the absence of the amino terminal variable region, the fragment was processed and translocated across the endoplasmic reticulum. The data suggest that the presumed recognition regions necessary for translocating eukaryotic secretory proteins across the endoplasmic reticulum do not reside in the amino terminal region near the signal peptide.