The Streptomyces reticuli alpha-chitin-binding protein CHB2 and its gene.

When co-cultivated with chitin-containing fungi, Streptomyces reticuli secretes the chitin-binding protein CHB2. Microscopical and immunological investigations revealed that CHB2 acts like a glue to mediate the contact between the fungal and the Streptomyces hyphae. CHB2 was purified to homogeneity, and the sequence of its N-terminal amino acids was determined and used to deduce an oligonucleotide, which was then used to probe a subgenomic library. The chb2 gene was cloned, sequenced and overexpressed. The deduced mature protein has a molecular mass of 18.6 kDa, and a large number of its amino acids are identical to those of CHB1 from Streptomyces olivaceoviridis. CHB2 effectively targets different types of alpha-chitin, but no other polysaccharide. The dissociation constant (Kd) for binding to purified crab shell chitin is 0.27 microM. Immunological studies suggest that homologues of CHB1 and CHB2 are secreted by streptomycetes while growing in the presence of alpha-chitin-containing substrates.