Chitin Accelerates Activation of a Novel Haloarchaeal Serine Protease That Deproteinizes Chitin-Containing Biomass

The haloarchaeonNatrinema sp. strain J7-2 has the ability to degrade chitin, and its genome harbors a chitin metabolism-related gene cluster that contains a halolysin gene,sptC . ThesptC gene encodes a precursor composed of a signal peptide, an N-terminal propeptide consisting of a core domain (N*) and a linker peptide, a subtilisin-like catalytic domain, a polycystic kidney disease domain (PkdD), and a chitin-binding domain (ChBD). Here we report that the autocatalytic maturation of SptC is initiated bycis -processing of N* to yield an autoprocessed complex (N*-IWT ), followed bytrans -processing/degradation of the linker peptide, the ChBD, and N*. The resulting mature form (MWT ) containing the catalytic domain and the PkdD showed optimum azocaseinolytic activity at 3 to 3.5 M NaCl, demonstrating salt-dependent stability. Deletion analysis revealed that the PkdD did not confer extra stability on the enzyme but did contribute to enzymatic activity. The ChBD exhibited salt-dependent chitin-binding capacity and mediated the binding of N*-IWT to chitin. ChBD-mediated chitin binding enhances SptC maturation by promoting activation of the autoprocessed complex. Our results also demonstrate that SptC is capable of removing proteins from shrimp shell powder (SSP) at high salt concentrations. Interestingly, N*-IWT released soluble peptides from SSP faster than did MWT . Most likely, ChBD-mediated binding of the autoprocessed complex to chitin in SSP not only accelerates enzyme activation but also facilitates the deproteinization process by increasing the local protease concentration around the substrate. By virtue of these properties, SptC is highly attractive for use in preparation of chitin from chitin-containing biomass.