A cystatin F homologue from large yellow croaker (Larimichthys crocea) inhibits activity of multiple cysteine proteinases and Ii chain processing in vitro

• A cystatin F homologue was identified and characterized in large yellow croaker.
• The LycCysF possesses unique structural and functional features as compared to mammalian cystatin F.
• The rLycCysF could not only inhibit the activity of multiple cysteine proteases, but also inhibit the Ii chain processing by recombinant cathepsin S in vitro.
• LycCysF may participate in the MHC-II associated antigen processing and presentation.

Cystatin F, a member of the family II cystatins, plays important roles in immune response-related processes through inhibiting specific enzyme targets. In this study, a cystatin F homologue, LycCysF, was identified and characterized from large yellow croaker (Larimichthys crocea). The deduced LycCysF protein exhibits a typical structural feature of type II cystatins, including three evolutionally conserved motifs, Gly35, QVVRG79−83 and PW130−131. Tissue expression analysis showed that LycCysF mRNA was expressed in all tissues examined, albeit at different levels. Recombinant LycCysF (rLycCysF) produced in Pichia pastoris could inhibit the activity of multiple cysteine proteases, including papain, legumain and recombinant large yellow croaker cathepsin B, L and S. Moreover, rLycCysF could inhibit the Ii chain processing by recombinant cathepsin S in vitro. These data suggest that LycCysF may participate in regulation of cathepsins and MHC-II associated Ii chain processing. In addition, mammalian cystatin F is produced as an inactive dimer, becoming activated by proteolysis in the endo/lysosome of immune cells and then exerts its function of regulating downstream proteases activity. However, the N-terminal extension and two additional cysteine residues responsible for dimer formation are absent in LycCysF and cystatin F from other fish species, reptiles and Aves, indicating that these proteins can not form dimer and may regulate the proteases activity via an alternate pathway distinct from mammalian cystatin F. To our knowledge, this is the first report on molecular characteristics of a teleost cystatin F and its role in Ii chain processing.