J. Biochem, 1998, Vol. 123, No. 6 1199-1207
© 1998 Japanese Biochemical Society
research-article |
Evidence for Involvement of Two Isoforms of Syk Protein-Tyrosine Kinase in Signal Transduction through the High Affinity IgE Receptor on Rat Basophilic Leukemia Cells
Department of Hygienic Chemistry, Faculty of Pharmaceutical Sciences, Hokkaido University Kita-12 Nishi-6, Kita-ku, Sapporo 060-0812
1To whom correspondence should be addressed. Tel: +81-11-7063245, Fax: +81-11-706-4989, E-mail: yamashit{at}pharm.hokudai.ac.jp
Recent evidence suggests a critical role for Syk in mast cell activation upon high affinity IgE receptor (Fc
RI) aggregation. A rat basophilic leukemia cell line, RBL-2H3, expresses similar levels of two Syk isoforms that differ with respect to the presence of a 23-amino acid insert within the "linker" region located between the second Src homology 2 and the catalytic domain. Although they exhibit comparable intrinsic enzymatic activity, functional differences between the two isoforms are unknown. Here we report that the deleted Syk isoform can mediate signal transduction in RBL-2H3 cells. Aggregation of chimeric kinase, consisting of either form of Syk fused to the transmembrane and extracellular domains of guinea pig type II IgG Fc receptor, on RBL transfectants resulted in degranulation, release of leukotrienes, and enhanced gene expression of tumor necrosis factor- a. The chimeras as well as phospholipase C-
1 and Vav became tyrosine-phosphorylated upon aggregation of chimeras. We also found that both Syk isoforms from transiently transfected COS-7 cells were capable of binding to phosphorylated FcRI, and their kinase activities were similarly up-regulated in the presence of tyrosine-phosphorylated synthetic peptides based on the sequence of the subunit of FcRI. Thus, these results establish that both isoforms of Syk can mediate signal transduction in mast cells and suggest that the 23-amino acid insert in the linker region of Syk may not be obligatory for FcRI signaling.
2L.K. is supported by the Hokkaido Overseas Association Inc. (Sapporo).
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