Characteristics of the surface-located carbohydrate-binding protein CbpC from Streptomyces coelicolor A3(2)

Streptomyces coelicolor A3(2) produces a 35.6-kDa carbohydrate-binding protein (named CbpC) in the presence of cellobiose, cellulose or chitin as sole carbon source. The protein was found secreted (a typical signal sequence was present at the N-terminus) and linked to the peptidoglycan layer of the mycelia. At its C-terminal end a putative cell-wall sorting signal was identified, consisting of (1) Streptomyces specific recognition site for a transpeptidase (LAETG instead of generic LPXTP consensus), (2) a hydrophobic region and (3) a tail of positively charged residues. The deletion of this sorting signal abolished the cell-wall attachment because the resulting CbpC-form was found extracellular. After purification this protein was shown to interact strongly with crystalline cellulose; different crystalline chitin-forms were recognised moderately and chitosan not. As demonstrated by analysing further truncated CbpC-forms a glycine-aspartate/serine rich region, which separates the carbohydrate-binding module from the sorting signal, plays an important role in protein stability.