Convergent evolution of processivity in bacterial and fungal cellulases.

Cellulose is the most abundant biomass on Earth, and many microorganisms depend on it as a source of energy. It consists mainly of crystalline and amorphous regions, and natural degradation of the crystalline part is highly dependent on the degree of processivity of the degrading enzymes (i.e., the extent of continuous hydrolysis without detachment from the substrate cellulose). Here, we report high-speed atomic force microscopic (HS-AFM) observations of the movement of four types of cellulases derived from the cellulolytic bacteria on various insoluble cellulose substrates. The HS-AFM images clearly demonstrated that two of them (Cel6B and Cel48A) slide on crystalline cellulose. The direction of processive movement of Cel6B is from the nonreducing to the reducing end of the substrate, which is opposite that of processive cellulase Cel7A of the fungus (Cel7A), whose movement was first observed by this technique, while Cel48A moves in the same direction as Cel7A. When Cel6B and Cel7A were mixed on the same substrate, "traffic accidents" were observed, in which the two cellulases blocked each other's progress. The processivity of Cel6B was similar to those of fungal family 7 cellulases but considerably higher than those of fungal family 6 cellulases. The results indicate that bacteria utilize family 6 cellulases as high-processivity enzymes for efficient degradation of crystalline cellulose, whereas family 7 enzymes have the same function in fungi. This is consistent with the idea of convergent evolution of processive cellulases in fungi and bacteria to achieve similar functionality using different protein foldings.