A LYMPHOCYTE-SPECIFIC LTK TYROSINE KINASE ISOFORM IS RETAINED IN THE ENDOPLASMIC-RETICULUM IN ASSOCIATION WITH CALNEXIN

A lymphocyte specific murine Ltk tyrosine kinase isoform was previousl y found to reside in the endoplasmic reticulum and to be potently acti vated upon treatment of cells with alkylating or thiol oxidizing agent s, Based on these observations, a unique role for Ltk was proposed as an endoplasmic reticulum-resident transmembrane kinase regulated by re dox changes (Bauskin, A. R., Alkalay, I., and Ben-Neriah, Y. (1991) Ce ll 66, 685-696), To analyze why this Ltk isoform is retained in the en doplasmic reticulum, we investigated its behavior in over-expressing c ells, Our results indicate that lymphoid Ltk exhibits a dual N-exo/C-c yt and N-cyt/C-exo transmembrane topology in transfected cells, This u nusual behavior may be responsible for retention in the endoplasmic re ticulum since mutants with an increased number of positive amino acids downstream of the transmembrane segment exhibit a conventional N-exo/ C-cyt orientation and proceed to the cell surface, Endoplasmic reticul um retained Ltk forms a prominent complex with the chaperone calnexin, suggesting that Ltk may be retained by the mechanism that prevents su rface expression of inappropriately folded proteins or incompletely as sembled protein complexes.