Proteasomal cleavage of substrate (26S proteasome catalyst)

Proteasomes are usually localized to the cytoplasm but are also present in the nucleus and ER. Houde et al. (2003) investigated the distribution of proteasomes in J774 macrophages and observed them on phagosomes. The proteasomes present on phagosomes may not come directly from ER, but instead assemble on phagosomes to play a function at a precise point during phagolysosomal biogenesis. Houde et al. also observed polyubiquitinated proteins on the cytoplasmic side of phagosomes and highlighted the link between the ubiquitination process and proteasomal degradation; translocated peptides are ubiquitinated and processed by the proteasome complex assembled on the cytoplasmic side of phagosomes, leading to the generation of MHC class I binding peptides.

蛋白酶体通常定位于细胞质中，但亦存在于细胞核和内质网。Houde 等人（2003年）研究了蛋白酶体在 J774 巨噬细胞中的分布，并观察到它们存在于噬体中。存在于噬体上的蛋白酶体并非直接源自内质网，而是于噬体上组装，并在噬体溶酶体生物发生过程中的特定阶段发挥功能。Houde 等人还观察到在噬体细胞质侧存在泛素化蛋白质，并强调了泛素化过程与蛋白酶体降解之间的联系；转移肽被泛素化，并由噬体细胞质侧组装的蛋白酶体复合体进行加工，从而生成 MHC 类 I 结合肽。