Human cathepsin W, a cysteine protease predominantly expressed in NK cells, is mainly localized in the endoplasmic reticulum

Human cathepsin W (also called lymphopain) is a recently described papain-l ike cysteine protease of unknown function whose gene expression was found t o be restricted to cytotoxic cells. Here we demonstrate that cathepsin W is expressed predominantly in NK cells and, to a lesser extent, in CTLs. Quan titative RT-PCR revealed that NK cells contained similar to 21 times more c athepsin W transcript than CTLs. The predominant expression of cathepsin W in NK cells was further confirmed by Western blot analysis and immunohistoc hemistry. IL-2-mediated stimulation of NK cells and CTLs revealed a stronge r up-regulation of the cathepsin W gene and protein expression in NK cells (7-fold) than in CTLs (2-fold). Transfection experiments of HeLa cells and biochemical analyses revealed that cathepsin W is exclusively "high mannose -type" glycosylated and is mainly targeted to the endoplasmic reticulum (ER ). Interestingly, the ER localization of cathepsin W was also found in NK c ells, in which colocalization studies revealed an overlapping staining of c athepsin W and Con A, an ER-specific lectin. Furthermore, subcellular fract ionation of cathepsin W-expressing cells confirmed the ER localization and showed that cathepsin W is membrane associated. Based on the results of thi s study, cathepsin W might represent a putative component of the ER-residen t proteolytic machinery. The constitutive expression in NK cells and the st ronger up-regulation of cathepsin W by IL-2 in NK cells than CTLs suggest t hat cathepsin W is not just a marker of cytotoxic cells but is, rather, spe cifically expressed in NK cells.