A novel class of secreted hydrophobic proteins is involved in aerial hyphae formation inStreptomyces coelicolorby forming amyloid-like fibrils

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Abstract
Streptomycetes exhibit a complex morphological differentiation. After a submerged mycelium has been formed, filaments grow into the air to septate into spores. A class of eight hydrophobic secreted proteins, ChpA–H, was shown to be instrumental in the development of Streptomyces coelicolor. Mature forms of ChpD–H are up to 63 amino acids in length, and those of ChpA–C are larger (±225 amino acids). ChpA–C contain two domains similar to ChpD–H, as well as a cell-wall sorting signal. The chp genes were expressed in submerged mycelium (chpE and chpH) as well as in aerial hyphae (chpA–H). Formation of aerial hyphae was strongly affected in a strain in which six chp genes were deleted (ΔchpABCDEH). A mixture of ChpD–H purified from cell walls of aerial hyphae complemented the ΔchpABCDEH strain extracellularly, and it accelerated development in the wild-type strain. The protein mixture was highly surface active, and it self-assembled into amyloid-like fibrils at the water–air interface. The fibrils resembled those of a surface layer of aerial hyphae. We thus conclude that the amyloid-like fibrils of ChpD–H lower the water surface tension to allow aerial growth and cover aerial structures, rendering them hydrophobic. ChpA–C possibly bind ChpD–H to the cell wall.