Spontaneous Cleavages of a Heterologous Protein, the CenA Endoglucanase of Cellulomonas fimi, in Escherichia coli

CenA is an endoglucanase secreted by the Gram-positive cellulolytic bacterium, Cellulomonas fimi, to the environment as a glycosylated protein. The role of glycosylation in CenA is unclear. However, it seems not crucial for functional activity and secretion since the unglycosylated counterpart, recombinant CenA (rCenA), is both bioactive and secretable in Escherichia coli. Using a systematic screening approach, we have demonstrated that rCenA is subjected to spontaneous cleavages (SC) in both the cytoplasm and culture medium of E. coli, under the influence of different environmental factors. The cleavages were found to occur in both the cellulose-binding (CellBD) and catalytic domains, with a notably higher occurring rate detected in the former than the latter. In CellBD, the cleavages were shown to occur close to potential N-linked glycosylation sites, suggesting that these sites might serve as ‘attributive tags’ for differentiating rCenA from endogenous proteins and the points of initiation of SC. It is hypothesized that glycosylation plays a crucial role in protecting CenA from SC when interacting with cellulose in the environment. Subsequent to hydrolysis, SC would ensure the dissociation of CenA from the enzyme-substrate complex. Thus, our findings may help elucidate the mechanisms of protein turnover and enzymatic cellulolysis.