Cohesin-dockerin interaction in cellulosome assembly - A single hydroxyl group of a dockerin domain distinguishes between nonrecognition and high affinity recognition

The assembly of enzyme components into the cellulosome complex is dictated by the cohesin-dockerin interaction. In a recent article (Mechaly, A, Yaron , S,, Lamed, R., Fierobe, H.-P., Belaich, A., Belaich, J.-P., Shoham, Y., a nd Bayer, E. A. (2000) Proteins 39, 170-177), we provided experimental evid ence that four previously predicted dockerin residues play a decisive role in the specificity of this high affinity interaction, although additional r esidues were also implicated. In the present communication, we examine furt her the contributing factors for the recognition of a dockerin by a cohesin domain between the respective cellulosomal systems of Clostridium thermoce llum and Clostridium cellulolyticum, In this context, the four confirmed re sidues were analyzed for their individual effect on selectivity, In additio n, other dockerin residues were discerned that could conceivably contribute to the interaction, and the suspected residues were similarly modified by site-directed mutagenesis, The results indicate that mutation of a single r esidue from threonine to leucine at a given position of the C. thermocellum dockerin differentiates between its nonrecognition and high affinity recog nition (K-a similar to 10(9) M-1) by a cohesin from C. cellulolyticum, This suggests that the presence or absence of a single decisive hydroxyl group is critical to the observed biorecognition, This study further implicates a dditional residues as secondary determinants in the specificity of interact ion, because interconversion of selected residues reduced intraspecies self -recognition by at least three orders of magnitude. Nevertheless, as the la tter mutageneses served to reduce but not annul the cohesin-dockerin intera ction within this species, it follows that other subtle alterations play a comparatively minor role in the recognition between these two modules.