Role of Molecular Chaperones in Ig Biosynthesis 11-2008

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致力于合成、组装和运输免疫球蛋白（IG）分子的工厂所必需的。这些异聚体蛋白在内质网（ER）中的产生是由一组称为分子伴侣的常驻ER蛋白辅助和监测的。如果IG蛋白不能正常成熟，它将被识别并穿过ER膜转移回细胞溶质，由26 S蛋白酶体降解。除了在浆细胞中每分钟处理大量的IG分子之外，产生抗体多样性的机制对B谱系细胞中的ER质量控制系统提出了进一步的要求。因此，该系统的许多组分首先在免疫细胞中被鉴定并不奇怪，并且ER质量控制装置的独特元件可能存在于这些细胞中。为了更好地定义控制IG蛋白的生物合成的机制，我们继续对分子伴侣BiP进行研究，该分子伴侣BiP结合游离的IG重链（HC）并阻止它们的转运，直到它们与轻链（LC）组装。我们假设BiP和它的辅因子以精心策划的方式一起工作，以帮助IG组装，监测该操作的成功，并最终靶向不正确折叠或组装的IG亚基进行降解。此外，我们假设ER的不同区域存在以适应蛋白质折叠和降解的看似拮抗的功能，并且单个ERdj家族成员允许BiP参与这些不同的功能。在本提案中，我们希望进一步描绘IG组装中的关键检查点，并确定执行这些检查点的机制。要做到这一点，我们将确定三个ER本地化的DnaJ同源物在IG折叠，装配和营业额的功能，定义的特异性核苷酸交换因子释放BiP从未折叠的蛋白质，并最终描绘用于识别未组装的IG分子和靶向它们的降解机制。公共卫生相关性：帮助和监测抗体折叠和组装的细胞过程对免疫系统的发育至关重要。此外，抗体一直是识别细胞质量控制机制组分的非常好的底物，并应继续为这一复杂过程提供新的见解。蛋白质折叠和ER质量控制的异常可能会产生破坏性后果，如在囊性纤维化、阿尔茨海默病和朊病毒疾病中观察到的。因此，更好地了解本提案中所研究的过程可能会产生更多的全球影响。

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 that occurs 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 these heteromeric proteins in the endoplasmic reticulum (ER) is both aided and monitored by a group of resident ER proteins known as molecular chaperones. If the Ig protein fails to mature properly, it is identified and transferred back across the ER membrane to the cytosol for degradation by the 26S proteasome. In addition to the massive number of Ig molecules that are processed each minute in a plasma cell, the mechanisms for generating antibody diversity put further demands on ER quality control systems in B lineage cells. Thus it is not surprising that many components of this system were first identified in immune cells, and it is possible that unique elements of the ER quality control apparatus could exist in these cells. To better define the mechanisms governing the biosynthesis of Ig proteins, we continue our studies on the molecular chaperone BiP, which binds to free Ig heavy chains (HC) and prevents their transport until they assemble with light chains (LC). We hypothesize that BiP and its co-factors 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. Furthermore, we hypothesize that distinct regions of the ER exist to accommodate the seemingly antagonistic functions of protein folding and degradation and that individual ERdj family members allow BiP to participate in these different functions. In the present proposal we wish to further delineate critical checkpoints in Ig assembly and determine the mechanisms by which they are executed. To do so, we will determine the function of three ER localized DnaJ homologues in Ig folding, assembly and turnover, define the specificity of nucleotide exchange factors in releasing BiP from unfolded proteins, and finally delineate mechanisms used to identify unassembled Ig molecules and target them for degradation. Public Health Relevance: Cellular processes that aid and monitor the folding and assembly of antibodies are crucial to the development of the immune system. In addition, antibodies have been unusually good substrates for identifying components of the cellular quality control machinery and should continue to provide new insights into this complex process. Abnormalities in protein folding and ER quality control can have devastating consequences as is observed in cystic fibrosis, Alzheimer's disease and prion diseases. Thus, a better understanding of the processes being investigated in this proposal is likely to have more global implications.