Tubulointerstitial nephritis antigen (MEROPS identifier C01.973) is an extracellular basement membrane protein originally identified as an antigen in anti-tubular basement membrane antibody-mediated interstitial nephritis. It is involved in renal tubulogenesis and is defective in hereditary disorders such as juvenile nephronophthisis. Human tubulointerstitial nephritis antigen is a homologue of the cysteine endopeptidase papain, but has the active site cysteine replaced by serine, and is therefore considered to be a non-peptidase homologue. The 160 amino acids N-terminal region is unrelated to the papain propeptide and instead includes a domain homologous to somatomedin B [].
