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Assembly and intracellular trafficking of the B cell antigen receptor in a mutant B cell lymphoma, WEHI 88.1 Escribano, Jessica

Abstract

Mutations that affect the synthesis of receptor subunits or that interfere with protein folding may lead to ER retention of unassembled receptors. This retention process is part of the secretory pathway's "quality control" mechanism, which consists of a variety of molecular chaperones and enzymes that assist nascent proteins in their folding, assembly, and post-translational modification. The B cell antigen receptor (BCR) is an oligomeric protein that undergoes a complex, stepwise, assembly process. The BCR consists of an antigen-binding component, mlgM (containing two immunoglobulin heavy (H) chains and two light (L) chains), and a signaling component, Ig-α/Ig-β. Both parts of the BCR must correctly assemble and associate with each other to leave the ER. If one of more subunits of the BCR is missing or defective, the entire receptor is retained and targeted for degradation (Matsuuchi et al., 1992). In the current study, we have characterized the BCR expressed by WEHI 88.1, a B lymphoma mutant. WEHI 88.1 expresses all four chains of the BCR, however, they are retained intracellularly in a pre-Golgi compartment. Our studies revealed that WEHI 88.1 mlgM assembly was defective, likely due to limiting amounts of K L chain. As a consequence of limiting amounts of K L chain, completely assembled mlgM composed of two H chains and two L chains (H2L2) was not detected in the mutant. In addition, pulse chase analysis indicated that K L chain turnover in the mutant differed from that in wild type cells. In contrast, the Ig-α/Ig-β part of the receptor assembled normally and associated with the mlgM present in the WEHI 88.1 cell line. This study has identified a key defect in the assembly of the BCR in mutated WEHI 88.1 cells and this mutant can serve as a tool for the future studies of receptor: chaperone interactions during the assembly process.

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