Role of Molecular Chaperones in Ig Biosynthesis 11-2008

描述(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戏剧性的变化之一。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。内质网(ER)中这些异构体蛋白的产生受到一组被称为分子伴侣的常驻ER蛋白的帮助和监控。如果Ig蛋白不能正常成熟，它就会被鉴定出来，并通过内质网膜转移回胞浆，由26S蛋白酶体降解。除了浆细胞中每分钟处理的大量Ig分子外，产生抗体多样性的机制对B系细胞中的ER质量控制系统提出了进一步的要求。因此，这一系统的许多成分首先在免疫细胞中被鉴定是不足为奇的，而且在这些细胞中可能存在内质网质量控制装置的独特成分。为了更好地确定免疫球蛋白生物合成的机制，我们继续研究分子伴侣Bip，它与自由的Ig重链(HC)结合，阻止它们的运输，直到它们与轻链(LC)组装。我们假设BiP和它的辅助因子以精心编排的方式共同作用，以帮助Ig组装，监测这一操作的成功，并最终针对错误折叠或组装的Ig亚基进行降解。此外，我们假设内质网的不同区域存在，以适应蛋白质折叠和降解的看似对立的功能，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，我们希望进一步划定免疫球蛋白装配中的关键检查点，并确定执行这些检查点的机制。为此，我们将确定三个ER定位的DNAJ同源物在Ig折叠、组装和周转中的功能，确定核苷酸交换因子在从未折叠的蛋白质中释放Bip的特异性，最后描述用于识别未组装的Ig分子和针对它们进行降解的机制。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展至关重要。此外，抗体一直是识别细胞质量控制机制组成部分的不同寻常的良好底物，应该继续为这一复杂过程提供新的见解。蛋白质折叠和内质网质量控制的异常可能会产生毁灭性的后果，就像在囊性纤维化、阿尔茨海默病和普恩病毒疾病中观察到的那样。因此，更好地理解这项提案中正在调查的进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.