RETROGRADE PROTEIN TRANSPORT IN RENAL EPITHELIAL CELLS

蛋白质跨膜转运的机制(S) 仍然是现代细胞生物学研究的中心焦点。单元格 膜被认为是一种选择性的蛋白质屏障，很少有 例外情况是，外源蛋白质要么被排除在细胞外，要么进入 一种以溶酶体降解为最终结果的“默认途径”。 外源蛋白质进入细胞内部是一种罕见的事件 到目前为止，似乎仅限于少数细菌毒素具有独一无二的 靶向那些设法逃脱溶酶体降解的序列。在……里面 与之前任何关于外源蛋白转位的描述不同， 我们最近在大鼠的近端发现了一种丰富的15.5 kDa蛋白质 α-微球蛋白的蛋白水解性裂解产物--小管 (A2)，一种19 kDa的蛋白质，主要由肝脏合成。A2是一种 蛋白质超家族的成员(包括视黄醇结合蛋白) 被认为是作为水解性配体的运输蛋白 作为长链脂肪酸。这种肝脏蛋白在体内的堆积 肾脏近端小管可能是生理学上第一个 “逆行蛋白质运输”的一个重要例子 自然合成的外源蛋白质积聚在胞浆中 属于另一种细胞类型。我们建议在肾小球滤过后， 近端小管细胞最终通过一条新的途径代谢A2 大量积累蛋白水解性裂解产物(A2-片段) 在细胞质中。为了评估这一假设，我们建议：(1) 描述A2由近端摄取和移位的过程 上皮细胞；(2)确定这一过程是否由 内吞作用(受体介导的、液相或吸附内吞作用) 或简单扩散；(3)评价A2与疏水分子的结合 配体是A2摄取所必需的或修饰的；(4)识别细胞 车厢(S)负责处理A2，使胞浆 A2发生积累，而不是降解，并且(5)确定 独特的结构特征是否决定了A2是如何由 近端小管细胞。这些研究可以为以下方面提供新的见解 一种以前未知的将蛋白质从外向内移动的途径 牢房。这一信息可能与定向交付相关 各种药剂或药物进入肾脏。此外，A2结合脂肪 在体外是一种脂肪酸结合蛋白，在体内则是一种“类脂肪酸结合蛋白”。 作为一种运输蛋白，A2可以调节脂肪酸氧化速率 在正常和病理生理状态下，如租房缺血。

The mechanism(s) by which proteins are translocated across membranes remains a central focus of research in modern cell biology. The cell membrane is thought to act as a selective protein barrier and with few exceptions, exogenous proteins are either excluded from the cell or enter a 'default pathway' that rapidly culminates with lysosomal degradation. Entry of an exogenous protein into the cell's interior is a rare event and thus far, appears restricted to a few bacterial toxins with unique targeting sequences that manage to escape lysosomal degradation. In contrast to any previous description of exogenous protein translocation, we recently found an abundant, 15.5 kDa protein in the rat proximal tubule that is a proteolytic cleavage product of alpha2mu-microglobulin (A2), a 19 kDa protein synthesized predominantly by the liver. A2 is a member of a protein superfamily (including retinol binding protein) thought to function as transport proteins for hydrolyphobic ligands such as long chain fatty acids. Accumulation of this hepatic protein within the proximal tubule of the kidney may be the first physiologically significant example of 'retrograde protein transport' in which a naturally synthesized, exogenous protein is accumulated in the cytosol of another cell type. We suggest that following glomerular filtration, the proximal tubule cell metabolizes A2 by a novel pathway, ultimately accumulating large amount of a proteolytic cleavage product (A2-fragment) in the cytosol. To evaluate this hypothesis, we propose to: (1) describe the process of A2 uptake and translocation by the proximal epithelial cell; (2) determine whether this process is mediated by endocytosis (receptor-mediated, fluid-phase, or adsorptive endocytosis) or simple diffusion; (3) evaluate whether binding of A2 to a hydrophobic ligand is required for, or modifies A2 uptake; (4) identify the cellular compartment(s) responsible for processing A2 so that cytosolic accumulation, rather than degradation, of A2 occurs and (5) determine whether unique structural features determine how A2 is processed by the proximal tubule cell. These studies could provide new insights regarding a previously unknown pathway for moving proteins from outside to inside the cell. This information could have relevance for targeted delivery of various agents or drugs to the kidney. In addition, A2 binds fatty acids in vitro and act as a 'fatty acid binding like-protein' in vivo. As a transport protein, A2 could modulate fatty acid oxidation rates during both normal and pathophysiologic states such as rental ischemia.