The B cell antigen receptor (BCR) is a multimeric complex which consists of membrane bound immunoglobulin (mIg) noncovalently with heterodimers of Ig-alpha and Ig-beta. It is these latter structures which translate antigen engagement into the cytoplasmic signalling events that drive cellular proliferation and differentiation. Ig-alpha and Ig-beta each have distinct, complementary and synergistic functions. In the context of tyrosine kinase activation, Ig-beta acts as an amplifier of the intrinsic ability of Ig-alpha to become phosphorylated and activate tyrosine kinases. The synergy between Ig-alpha and Ig-beta translates into qualitative differences in the abilities of receptor complexes containing both to facilitate cellular processes such as apoptosis and the presentation of antigen to T cells. Presentation requires the integration of chain specific signalling and trafficking capacities. Based on the above observations, we propose two central hypotheses: 1. Ig-beta enhances the qualitative signalling capacities of Ig-alpha and, by extension, that of the intact receptor complex. 2. The signalling pathways activated synergistically by the Ig-alpha/beta heterodimer are necessary for receptor functions including B cell development and receptor facilitated antigen presentation. We propose to test these hypotheses in vivo and to determine the mechanisms of cooperativity between Ig-alpha and Ig-beta in the following Specific Aims:
Aim 1. To determine the molecular and biochemical mechanisms mediating Ig- alpha/beta synergy. We will construct a series of switch and point mutants of Ig-alpha and Ig-beta which will be used to elucidate the, domains within Ig-beta which enhance Ig-alpha tyrosine phosphorylation and to determine the location and consequences of this enhancement in terms of effector recruitment and signalling pathway activation.
Aim 2. To determine the contributions of Ig-alpha and Ig-beta to B cell development. We have derived transgenic mice expressing chimeras containing the cytoplasmic domains of Ig-alpha and/or Ig-beta. They will be used to study the contributions of Ig-alpha/beta to the transition of specific developmental checkpoints.
Aim 3. To elucidate the signalling and trafficking mechanisms utilized by the BCR to enhance antigen presentation. We propose to use the mutants described in Aim I to determine which signalling pathways are involved in antigen presentation and furthermore, which targeting sequences in Ig-alpha/beta are required for trafficking to the antigen processing compartment. We also will utilize the transgenic mice described in Aim 2 to elucidate the contributions of Ig-alpha/beta to antigen presentation by native B cells.
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