Cathepsin L (Major excreted protein, MEP)

Cathepsin L is a lysosomal endopeptidase belonging to the papain cysteine protease superfamily. Elevated levels of cathepsin L have been found in primary cell cultures from cancerous prostatic samples and in synovial lining from rheumatoid arthritis and osteoarthritis samples. Cathepsin L was found to be necessary for Li (invariant chain) degradation, a critical step in major histocompatibility complex class II – restricted antigen presentation, in cortical thymic epithelial cells, but not in bone marrow derived antigen – presenting cells.