Cathepsin L1 is a member of the Peptidase C1 (cathepsin) MEROPS family, which plays an important role in diverse processes including normal lysosome mediated protein turnover, antigen and proprotein processing, and apoptosis.[10] Its substrates include collagen and elastin, as well as alpha-1 protease inhibitor, a major controlling element of neutrophil elastase activity. The encoded protein has been implicated in several pathologic processes, including myofibril necrosis in myopathies and in myocardial ischemia, and in the renal tubular response to proteinuria. This protein, which is a member of the peptidase C1 family, is a dimer composed of disulfide-linked heavy and light chains, both produced from a single protein precursor. At least two transcript variants encoding the same protein have been found for this gene.[5]
Viral entry
Cleavage of the SARS-CoV-2S2spike protein required for viral entry into cells can be accomplished by proteasesTMPRSS2 located on the cell membrane, or by cathepsins (primarily cathepsin L) in endolysosomes.[11]Hydroxychloroquine inhibits the action of cathepsin L in endolysosomes, but because cathepsin L cleavage is minor compared to TMPRSS2 cleavage, hydroxychloroquine does little to inhibit SARS-CoV-2 infection.[11]
Inflammation
Although Cathepsin L is usually characterized as a lysosomal protease, it can be secreted, resulting in pathological inflammation.[12] Cathepsin L and other cysteine cathepsins tend to be secreted by macrophages and other tissue-invading immune cells when causing pathological inflammation.[13]
^Dickinson DP (2002). "Cysteine Peptidases of Mammals: Their Biological Roles and Potential Effects in the Oral Cavity and Other Tissues in Health and Disease". Critical Reviews in Oral Biology and Medicine. 13 (3): 238–75. doi:10.1177/154411130201300304. PMID12090464.
^Majerle A, Jerala Roman (Sep 2003). "Protein inhibitors form complexes with procathepsin L and augment cleavage of the propeptide". Arch. Biochem. Biophys. 417 (1): 53–8. doi:10.1016/S0003-9861(03)00319-9. ISSN0003-9861. PMID12921779.
^Estrada S, Nycander M, Hill N J, Craven C J, Waltho J P, Björk I (May 1998). "The role of Gly-4 of human cystatin A (stefin A) in the binding of target proteinases. Characterization by kinetic and equilibrium methods of the interactions of cystatin A Gly-4 mutants with papain, cathepsin B, and cathepsin L". Biochemistry. 37 (20): 7551–60. doi:10.1021/bi980026r. ISSN0006-2960. PMID9585570.
Stearns NA, Dong JM, Pan JX, et al. (1991). "Comparison of cathepsin L synthesized by normal and transformed cells at the gene, message, protein, and oligosaccharide levels". Arch. Biochem. Biophys. 283 (2): 447–57. doi:10.1016/0003-9861(90)90666-M. PMID2275556.
Schick C, Pemberton PA, Shi GP, et al. (1998). "Cross-class inhibition of the cysteine proteinases cathepsins K, L, and S by the serpin squamous cell carcinoma antigen 1: a kinetic analysis". Biochemistry. 37 (15): 5258–66. doi:10.1021/bi972521d. PMID9548757.
Estrada S, Nycander M, Hill NJ, et al. (1998). "The role of Gly-4 of human cystatin A (stefin A) in the binding of target proteinases. Characterization by kinetic and equilibrium methods of the interactions of cystatin A Gly-4 mutants with papain, cathepsin B, and cathepsin L.". Biochemistry. 37 (20): 7551–60. doi:10.1021/bi980026r. PMID9585570.