Protein kinase C (PKC) beta has been reported (Shinohara, H., T. Yasuda, Y. Aiba, H. Sanjo, M. Hamadate, H. Watarai, H. Sakurai, and T. Kurosaki. 2005. J. Exp. Med. 202:1423-1431; Sommer, K., B. Guo, J.L. Pomerantz, A.D. Bandaranayake, M.E. Moreno-Garcia, Y.L. Ovechkina, and D.J. Rawlings. 2005. Immunity. 23:561-574) to play a crucial role in B cell receptor (BCR)-mediated IkappaB kinase (IKK) activation through phosphorylation of caspase recruitment domain 11, Bimp3 (CARMA1). However, it remains unclear whether this PKCbeta-mediated phosphorylation accounts fully for the activation status of CARMA1, because involvement of other kinases, such as phosphoinositide 3-kinase-dependent kinase 1, has also been suggested. We show that PKCbeta mediates phosphorylation of CARMA1 on Ser668, which in turn is essential for BCR-mediated CARMA1-Bcl10-mucosal-associated lymphoid tissue 1 (MALT1) association and subsequent IKK activation. Our analyses also demonstrate that the downstream kinase IKKbeta contributes to facilitating formation of the complex CARMA1-Bcl10-MALT1 by mediating phosphorylation of CARMA1. Hence, our data suggest that PKCbeta is crucial for initial activation of IKK. The activated IKKbeta does not merely function as an effector enzyme but also modifies the upstream signaling complex through a feedback mechanism, thereby optimizing the strength and duration of the nuclear factor kappaB signal.