Fig. 5: BBP binds to chitin.
a The appearance of samples at the end of the incubation of BBP(βCar), OCP2(ECH), or BSA in the absence or in the presence of chitin. Chitin in the SEC buffer instead of these proteins is shown as a control. b The insoluble chitin fraction incubated in the presence of BBP is intensely colored yellow. The sedimented fractions are shown. c SDS-PAGE analysis of the results of the chitin-binding assay. Pre-incubated chitin-protein complexes were sedimented and then washed with SEC buffer, 2 M urea and SDS, consecutively. Control lanes show that chitin on its own contains a negligible amount of proteins. M designates protein standards with the known Mw values (indicated in kDa). The apparent Mw of BBP (no tag), OCP2 and BSA are shown by arrows. d Structures of chitin, chitosan and murein fragments. The N-acetyl groups tentatively used for BBP attachment are highlighted by green. R - tetrapeptide of murein. e Schematic showing the presumed mechanism of BBP(βCar) complexes accumulation in pigment granules and cuticular chitin coloration by secretion of BBP(βCar) representing the yellow pigment (left). RER - rough endoplasmic reticulum. Schematic showing BBP(βCar) reconstitution in E. coli, non-specific attachment to the murein component of cell debris after lysis, and extraction by 2 M urea, which disrupts BBP(βCar)-murein interactions (right).
