ACTIVATION OF AP-1 IN PRIMARY B-LYMPHOCYTES BY SURFACE-IMMUNOGLOBULINREQUIRES DE-NOVO JUN-B SYNTHESIS

We demonstrate herein that resting primary B lymphocytes do not contai n detectable levels of AP-1 (TRE)-binding activity. Upon cross-linking of surface immunoglobulin (sIg) receptors, TRE-binding activity is in duced within 2 hr and its appearance requires de novo protein synthesi s. Antisera to Jun-B inhibits the vast majority of TRE-binding activit y, indicating that Jun-B is a primary component of B cell TRE-binding complexes. In keeping with this, Jun-B protein is not detectable in cy tosol or nuclear extracts from resting B lymphocytes, as determined by immunoblotting with Jun-B antisera. However, the nuclear expression o f Jun-B is induced within 2 hr following sig cross-linking and is comp letely blocked by cycloheximide. S-35-labeling studies suggest that th e increase in Jun-B expression results from de nova protein synthesis. Moreover, Jun-B migrates in SDS-polyacrylamide gels as two distinct e lectrophoretic proteins that correspond to a 41-kDa species and a phos phorylated 47-kDa form. These results suggest that the induction of AP -1-binding activity in primary B lymphocytes following sig cross-linki ng does not result from post-translational phosphorylation of a preexi sting cellular pool of Jun-B protein, but rather is coupled to the sti mulation of de novo Jun-B synthesis. Thus Jun-B synthesis represents a n integral event in the production of receptor-mediated AP-1 in B cell s. The significance of these results with respect to the role of Jun-B in controlling gene expression during the activation of primary B cel ls is discussed. (C) 1994 Academic Press, Inc.