Cuervo A M, Palmer A, Rivett A J, Knecht E
Instituto de Investigaciones Citológicas, Fundación Valenciana de Investigaciones Biomédicas, Spain.
Eur J Biochem. 1995 Feb 1;227(3):792-800. doi: 10.1111/j.1432-1033.1995.tb20203.x.
Proteasomes are high-molecular-mass multisubunit complexes which are believed, either by themselves or as a part of the 26S proteinase complex, to play a central role in extralysosomal pathways of intracellular protein breakdown. We have addressed the degradation of proteasomes in rat liver, investigating the possible role of lysosomes. Affinity-purified antibodies against rat liver proteasomes were used for immunoblot analysis of isolated lysosomes. Although proteasomes are not found in lysosomes from normally fed rats, they were found to accumulate in lysosomes of rats treated with leupeptin (an inhibitor of lysosomal proteases) and could also be detected in lysosomes isolated from livers of starved (24 h) rats. Proteinase-K treatment of these fractions, as well as immunogold procedures, show that a proportion of the proteasomes are inside lysosomes. Comparison of the amount of proteasomes found in lysosomes by immunoblotting with their experimentally determined half life (8.3 days) is consistent with an important role of these organelles in the degradation of rat liver proteasomes. Nevertheless, these data do not exclude the possibility that some nonlysosomal degradation of proteasome components also occurs. Since proteasomes were localized in autophagic vacuoles, it is likely that they are taken up mainly by nonselective autophagy. However, using an in vitro system, it was found that, under conditions of starvation, proteasomes may also be taken up into lysosomes and degraded via the heat-shock cognate protein of 73 kDa (hsc73)-mediated transport.
蛋白酶体是高分子质量的多亚基复合物，人们认为其自身或作为26S蛋白酶复合物的一部分，在细胞内蛋白质分解的溶酶体外途径中发挥核心作用。我们研究了大鼠肝脏中蛋白酶体的降解情况，探讨了溶酶体可能发挥的作用。用针对大鼠肝脏蛋白酶体的亲和纯化抗体对分离出的溶酶体进行免疫印迹分析。虽然在正常喂食大鼠的溶酶体中未发现蛋白酶体，但在经亮抑酶肽（一种溶酶体蛋白酶抑制剂）处理的大鼠溶酶体中发现它们会积累，并且在饥饿（24小时）大鼠肝脏分离出的溶酶体中也能检测到。对这些组分进行蛋白酶K处理以及免疫金程序表明，一部分蛋白酶体存在于溶酶体内。通过免疫印迹法测定溶酶体中蛋白酶体的量，并与其实验确定的半衰期（8.3天）进行比较，这与这些细胞器在大鼠肝脏蛋白酶体降解中发挥的重要作用是一致的。然而，这些数据并不排除蛋白酶体成分也存在一些非溶酶体降解的可能性。由于蛋白酶体定位于自噬泡中，它们很可能主要通过非选择性自噬被摄取。然而，利用体外系统发现，在饥饿条件下，蛋白酶体也可能被摄取到溶酶体中，并通过73 kDa热休克同源蛋白（hsc73）介导的转运进行降解。
