A surrogate 15 kDa JC kappa protein is expressed in combination with mu heavy chain by human B cell precursors.

A novel kappa protein, encoded by a germline JC kappa transcript, is expressed by normal and leukemic human B cell precursors. The transcript displays an open reading frame initiated by a non-AUG codon, and predicts a 15 kDa molecule which could be readily confirmed by in vitro translation. Cellular expression was demonstrated by immunofluorescence, precipitation and Western blotting. Furthermore, 2-D gel electrophoresis revealed that germline JC kappa can covalently associate with mu heavy chain at the surface of pre-B cells. We therefore propose that during B cell lymphopoiesis, two alternative pathways could be operative in which mu heavy chain can either associate with lambda 5 or germ-line JC kappa.