Utano Tomaru, Akihiro Ishizu, Shigeo Murata, Yukiko Miyatake, Sayuri Suzuki, Satomi Takahashi, Taku Kazamaki, Jiro Ohara, Tomohisa Baba, Sari Iwasaki, Kazunori Fugo, Noriyuki Otsuka, Keiji Tanaka, Masanori Kasahara
BLOOD 113 (21) 5186 - 5191 0006-4971 2009/05
[Refereed][Not invited] The ubiquitin-proteasome pathway, which degrades intracellular proteins, is involved in numerous cellular processes, including the supply of immunocompetent peptides to the antigen presenting machinery. Proteolysis by proteasomes is conducted by three beta subunits, beta 1, beta 2, and beta 5, of the 20S proteasome. Recently, a novel beta subunit expressed exclusively in cortical thymic epithelial cells was discovered in mice. This subunit, designated beta 5t, is a component of the thymoproteasome, a specialized type of proteasomes implicated in thymic positive selection. In this study, we show that, like its mouse counterpart, human beta 5t is expressed exclusively in the thymic cortex. Human beta 5t was expressed in approximately 80% of cortical thymic epithelial cells and some cortical dendritic cells. Human beta 5t was incorporated into proteasomes with two other catalytically active beta subunits beta 1i and beta 2i, forming 20S proteasomes with subunit compositions characteristic of thymoproteasomes. The present study demonstrates, for the first time, the existence of thymoproteasomes in the human thymic cortex, indicating that thymoproteasome function is likely conserved between humans and mice. (Blood. 2009; 113: 5186-5191)