Vol. 83 Nos. 9-10 (2007)

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A structural model indicating the interaction of CprB, an A-factor receptor homologue, with its ligand and target DNA.

CprB is a homologue of the receptor protein ArpA for A-factor, a hormonal γ-butyrolactone (GB) regulator in Streptomyces griseus. ArpA acts as a repressor for adpA encoding a key transcriptional activator for multiple genes involved in secondary metabolism and morphological differentiation. Binding of A-factor to ArpA causes its dissociation from the adpA promoter to release the repressive effects, leading to streptomycin production, spore formation and many other phenomena. CprB is a homo-dimeric protein, one subunit of which is illustrated with a ribbon model and the other with a tube model. A-factor embedded in the ligand-binding pocket is indicated with a ball model, while the dimer is indicated in a form allowing binding to its target DNA (with a stick model). Helices 1 (red), 2 (orange), and 3 (yellow) form a helix-turn-helix DNA-binding motif. A long helix 4 (green), connecting the ligand-binding domain and the DNA-binding domain, transfers the structural shift induced by binding of A-factor to the DNA-binding domain so that the DNA-binding motif relocates outside and dissociates the protein from the DNA.
GB molecules are widely distributed in Streptomyces species and are involved in the regulation of secondary metabolism and morphological differentiation. However, the effects of GB on antibiotic production and spore formation are different from species to species. In the complex regulatory cascades starting from GB, the receptor proteins play a critical role, because they repress the expression of global transcriptional activators such as adpA in S. griseus and SARP (Streptomyces antibiotic regulatory protein) genes in other Streptomyces species. The crystal structure of CprB suggests the mechanism how the GB receptor protein is released from the target DNA by binding of GB. The same mechanism may function in other GB-dependent regulatory systems in Streptomyces.

Haruyasu Kinashi

Department of Molecular Biotechnology

Graduate School of Advanced Sciences of Matter

Hiroshima University