We have identified a Ser/Thr kinase associated with the B cell receptor (BCR) complex as protein kinase C μ (PKCμ). PKCμ activity is up-regulated after crosslinking the BCR and CD19 on B cells, and PKCμ co-precipitates with Syk and phospholipase C-γ1/2 (PLCγ1/2). In vitro phosphorylation of fusion proteins showed that both Syk and PLCγ1 are potential substrates of PKCμ in vivo. Analysis of mutants of the chicken B cell line DT40 deficient in either Syk, Lyn, Btk, or PLCγ2 revealed that BCR-induced activation of PKCμ, like activation of PLCγ2, requires Syk and is partially regulated by Btk, but is Lyn independent. PKCμ can down-regulate the ability of Syk to phosphorylate PLCγ1 in vitro. Thus, PKCμ may function in a negative feedback loop regulating BCR-initiated signaling cascades.
Sidorenko, S. P., Law, C. L., Klaus, S. J., Chandran, K. A., Takata, M., Kurosaki, T., & Clark, E. A. (1996). Protein kinase C μ (PKCμ) associates with the B cell antigen receptor complex and regulates lymphocyte signaling. Immunity, 5(4), 353–363. https://doi.org/10.1016/S1074-7613(00)80261-7