Characterization of a novel, antifungal, chitin-binding protein from Streptomyces tendae Tu901 that interferes with growth polarity

The afp1 gene, which encodes the antifungal protein AFP1, was cloned from n ikkomycin-producing Streptomyces tendae Tu901, using a nikkomycin-negative mutant as a host and screening transformants for antifungal activity agains t Paecilomyces variotii in agar diffusion assays. The 384-bp afp1 gene has a low G+C content (63%) and a transcription termination structure with a po ly(T) region, unusual attributes for Streptomyces genes. AFP1 was purified from culture filtrate of S. tendae carrying the afp1 gene on the multicopy plasmid pIJ699. The purified protein had a molecular mass of 9,862 Da and l acked a 12-residue N-terminal peptide deduced from the nucleotide sequence. AFP1 was stable at extreme pH values and high temperatures and toward comm ercial proteinases. AFP1 had limited similarity to cellulose-binding domain s of microbial plant cell wall hydrolases and bound to crab shell chitin, c hitosan, and cell walls of P. variotii but showed no enzyme activity. The b iological activity of AFP1, which represents the first chitin-binding prote in from bacteria exhibiting antifungal activity, was directed against speci fic ascomycetes, and synergistic interaction with the chitin synthetase inh ibitor nikkomycin inhibited growth of Aspergillus species. Microscopy studi es revealed that fluorescein-labeled AFP1 strongly bound to the surface of germinated conidia and to tips of growing hyphae, causing severe alteration s in cell morphogenesis that gave rise to large spherical conidia and/or sw ollen hyphae and to atypical branching.