Richards J D, Gold M R, Hourihane S L, DeFranco A L, Matsuuchi L
Department of Microbiology and Immunology, G.W. Hooper Foundation, University of California, San Francisco 94143,USA.
J Biol Chem. 1996 Mar 15;271(11):6458-66. doi: 10.1074/jbc.271.11.6458.
B cell antigen receptor (BCR) cross-linking activates both Src family and Syk tyrosine kinases, resulting in increased cellular protein-tyrosine phosphorylation and activation of several downstream signaling enzymes. To define the role of Syk in these events, we expressed the BCR in the AtT20 mouse pituitary cell line. These nonlymphoid cells endogenously expressed the Src family kinase Fyn but not Syk. Anti-IgM stimulation of these cells failed to induce most of the signaling events that occur in B cells. BCR-expressing AtT20 transfectants were generated that also expressed Syk. Syk expression reconstituted several signaling events upon anti-IgM stimulation, including Syk phosphorylation and association with the BCR, tyrosine phosphorylation of numerous proteins including Shc, and activation of mitogen-activated protein kinase. In contrast, Syk expression did not reconstitute anti-IgM-induced inositol phosphate production. A catalytically inactive Syk mutant could associate with the BCR and become tyrosine phosphorylated but could not reconstitute downstream signaling events. Expression of the Src family kinase Lck instead of Syk also did not reconstitute signaling. Thus, wild type Syk was required to reconstitute several BCR-induced signaling events but was not sufficient to couple the BCR to the phosphoinositide signaling pathway.
B细胞抗原受体（BCR）交联可激活Src家族和Syk酪氨酸激酶，导致细胞蛋白酪氨酸磷酸化增加，并激活几种下游信号酶。为了确定Syk在这些事件中的作用，我们在AtT20小鼠垂体细胞系中表达了BCR。这些非淋巴细胞内源性表达Src家族激酶Fyn，但不表达Syk。用抗IgM刺激这些细胞未能诱导B细胞中发生的大多数信号事件。构建了表达BCR的AtT20转染细胞系，这些细胞也表达Syk。抗IgM刺激后，Syk的表达重建了几种信号事件，包括Syk磷酸化及其与BCR的结合、包括Shc在内的多种蛋白质的酪氨酸磷酸化，以及丝裂原活化蛋白激酶的激活。相比之下，Syk的表达并未重建抗IgM诱导的肌醇磷酸生成。一种催化失活的Syk突变体可以与BCR结合并发生酪氨酸磷酸化，但不能重建下游信号事件。表达Src家族激酶Lck而非Syk也不能重建信号。因此，野生型Syk是重建几种BCR诱导的信号事件所必需的，但不足以将BCR与磷酸肌醇信号通路偶联。
