The analysis of B cell antigen receptor (BCR) mutants deficient in antigen-independent signaling reveals novel BCR-associated proteins

The B cell antigen receptor (BCR) is composed of the membrane-bound immunoglobulin (mIg) <br>and the Ig-alpha/Ig-beta heterodimer and plays a central role in development, survival, and activation <br>of B lymphocytes. During B cell development, the BCR seems to be able to generate signals in <br>an antigen-dependent and -independent fashion. Upon binding to antigen, the BCR emits a <br>negative selection signal for immature and an activation signal for mature B cells. In the <br>absence of antigen, the BCR signals positive selection of immature and maintenance for mature <br>B cells. The BCR-dependent, antigen-independent signaling mode can be analyzed by <br>stimulating B cells with the phosphatase inhibitor pervanadate (PV). Analysis of substrate <br>tyrosine phosphorylation after PV-treatment of different B cell lines expressing BCR mutants <br>revealed drastic differences in phosphorylation of two signaling proteins namely the SH2- <br>domain-containing inositolpolyphosphate 5&#8217;-phosphatase (SHIP) and the protein kinase C delta. <br>Tyrosine phosphorylation of SHIP in response to PV stimulation was critically dependent on <br>the integrity of the Ig-alpha C-terminus. SHIP could be phosphorylated by both BCR-proximal <br>protein tyrosine kinases, Lyn and Syk. The phosphorylation of SHIP by Syk, but not that by <br>Lyn, was strictly dependent on the presence of an integer BCR complex, indicating that Syk <br>requires BCR expression for the phosphorylation of SHIP. Furthermore, the absence of proper <br>SHIP activation led to a disturbed balance of PKB and MAPK pathway activation, suggesting <br>that SHIP is a crucial regulator of the antigen-independent maintenance signal in B cells. <br>In contrast to SHIP, PKC-delta was able to bind in vitro to phosphorylated peptides derived from <br>the Ig-alpha C-terminus. As observed by native gel electrophoresis PKC-delta can be purified with the <br>BCR complex. The PKC-delta/BCR binding was dependent on the C-terminus of Ig-alpha. Moreover, <br>by confocal studies, PKC-delta was found to influence BCR/Syk localization, probably by inducing <br>BCR/Syk internalization. These data suggest, together with published data from PKC-delta <br>deficient mice that PKC-delta can terminate BCR signaling by binding to the BCR.

Author dc:creator

• Pracht, Catrin

Contributors dc:contributor

• Reth, Michael

• Tyrosin Phosphorylierung
• tyrosine phosphorylation
• transducer complex

Repository record source_url

https://freidok.uni-freiburg.de/data/1561

OAI identifier oai:identifier

oai:freidok.uni-freiburg.de:1561