Role of Molecular Chaperones in Ig Biosynthesis

描述（由申请人提供）：B细胞向浆细胞的分化代表了已知细胞结构中最显著的变化之一。分泌途径的大量增加是使浆细胞成为专门合成、组装和运输免疫球蛋白（Ig）分子的工厂所必需的。成熟抗体的产生是由一组称为分子伴侣的内质网蛋白辅助和监测的。这些蛋白的内质网浓度由未折叠蛋白反应（UPR）途径调节，以适应细胞对分泌途径蛋白产生的需求。最近的研究表明，UPR在浆细胞分化过程中被激活，而XBP-1转录因子是这一途径的一个组成部分，对浆细胞的产生至关重要。有充分的证据表明，内质网伴侣BiP与未组装的Ig重链结合，并防止其过早运输。从未折叠的底物中释放BiP是一个受到ATP严格控制的过程。我们已经确定了三种调节BiP的atp酶活性的新蛋白，因此可能参与BiP从Ig底物的释放。其中两个，ERdj3和ERdj4，是DnaJ的同源物，可以增加BiP的ATP水解速率，并且可以稳定BiP与未折叠的Ig重链底物的结合。第三种蛋白，BAP，是唯一已知的BiP核苷酸交换因子，应该促进BiP从底物的释放。这三种蛋白在B细胞分化过程中均上调。ERdj3和ERdj4都是XBP-1转录因子的靶标，ERdj3直接结合到未组装的Ig重链上。因此，我们假设这些BiP调节因子可能是B细胞最终分化所必需的。最后，我们发现内质网定位的Herp蛋白，包含一个泛素样结构域，可能参与靶向内质网蛋白的降解，在UPR和浆细胞分化过程中被上调。我们假设这些不同的蛋白质以一种精心安排的方式协同工作，以帮助Ig组装，监测这一操作的成功，并最终针对不正确折叠或组装的Ig亚基进行降解。我们建议结合生化、细胞培养和遗传实验来确定各种BiP调节因子在控制Ig合成和B细胞向浆细胞分化中的作用和要求。

DESCRIPTION (provided by applicant): The differentiation of a B cell to a plasma cell represents one of the most dramatic changes in cellular architecture known. The massive increase in the secretory pathway is necessary to allow the plasma cell to become a factory dedicated to the synthesis, assembly and transport of immunoglobulin (Ig) molecules. The production of mature antibodies is aided and monitored by a group of resident ER proteins known as molecular chaperones. The ER concentration of these proteins is regulated by the unfolded protein response (UPR) pathway to accommodate cellular demands for the production of secretory pathway proteins. Recent studies revealed that the UPR is activated during plasma cell differentiation, and that the XBP-1 transcription factor, which is a component of this pathway, is essential for the production of plasma cells. It has been well-documented that the ER chaperone BiP binds to unassembled Ig heavy chains and prevents their premature transport. The release of BiP from unfolded substrates is a tightly controlled process that is regulated by ATP. We have identified three novel proteins that regulate BiP's ATPase activity and are therefore likely to be involved in the release of BiP from Ig substrates. Two of these, ERdj3 and ERdj4, are DnaJ homologues and increase BiP's rate of ATP hydrolysis and should serve to stabilize BiP's binding to unfolded Ig heavy chain substrates. The third protein, BAP, is the only known nucleotide exchange factor for BiP and should enhance the release of BiP from substrates. All three proteins are up-regulated during B cell differentiation. Both ERdj3 and ERdj4 are targets of the XBP-1 transcription factor, and ERdj3 is bound directly to unassembled Ig heavy chains. Thus, we hypothesize these BiP regulators may be required for terminal differentiation of B cells. Lastly, we have found that the ER-localized Herp protein, which contains a ubiquitin-like domain and may be involved in targeting ER proteins for degradation, is up-regulated by the UPR and during plasma cell differentiation. We hypothesize that these different proteins work together in a carefully orchestrated fashion to aid Ig assembly, monitor the success of this operation, and finally to target improperly folded or assembled Ig subunits for degradation. We propose a combination of biochemical, cell culture, and genetic experiments to determine the roles and requirements of the various BiP regulators in controlling Ig synthesis and the differentiation of B cells to plasma cells.