Regulation of B cell receptor-induced calcium signaling through differential phosphorylation of Syk

Julie Jung Hong, Purdue University

Abstract

Signaling through the B cell antigen receptor can result in a variety of physiological responses, including apoptosis, receptor editing, induction of anergy, proliferation and differentiation. Although it is known that the physiological response that ensues is determined by changes in calcium concentration that are induced by antigen binding to the BCR, the mechanism by which BCR ligation leads to distinct calcium responses is currently not understood. The protein tyrosine kinase, Syk, is essential to BCR-stimulated calcium signaling and it can bind to both stimulatory and inhibitory molecules through interactions with phosphotyrosines located in its linker region. We analyzed the ability of different site-directed mutants of Syk, in which individual tyrosines were replaced with phenylalanines, to couple the BCR to downstream events that regulate the calcium response. Syk that is not phosphorylated on Y317, Y342, nor Y346 couples the BCR to calcium mobilization through a PI3K-dependent pathway, since a form of Syk that lacks any of these linker region tyrosines couples the BCR to the phosphorylation of PLCγ, IP3 production, calcium mobilization, and NFAT activation only in the presence of active PI3K. The phosphorylation of both Y342 and Y346 creates a docking site for the C-terminal SH2 domain of PLCγ, is required for its phosphorylation and activation, enhances the early phase of calcium mobilization, and provides a PI3K-independent pathway for calcium flux. Elimination of Y317 dramatically enhances these signals, and thus, a major mode of regulation occurs through the phosphorylation of Y317, which strongly inhibits both phases of the calcium response. The dual nature of the Src family kinase, Lyn, in BCR-induced calcium signaling is apparent from our results as it inhibits by phosphorylating Y317 of Syk, but also stimulates by activating the PI3K-dependent pathway. The BCR-induced calcium signaling that occurs in the absence of Lyn is, therefore, attributed to the presence of Y342 and Y346 of Syk. Thus, it is evident that Syk and Lyn cooperate to determine the pathway through which the BCR is coupled to calcium mobilization, which consequently affects both the magnitude and duration of the calcium response and ultimately determines the physiological outcome of receptor engagement.

Degree

Ph.D.

Advisors

Geahlen, Purdue University.

Subject Area

Molecular biology|Biochemistry|Cellular biology

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