CHARACTERIZATION OF THE SUBUNITS IN AN APPARENTLY HOMOGENEOUS SUBPOPULATION OF CLOSTRIDIUM-THERMOCELLUM CELLULOSOMES

Clostridium thermocellum cellulosomes isolated by cellulose affinity c hromatography were fractionated by anion exchange chromatography into apparently homogeneous subpopulations that differed with respect to en zyme activity and subunit composition. One such subpopulation containe d predominantly six subunits and was closely similar to the ''subcellu losome'' described by Kobayashi et al. (Kobayashi, T., Romaniec, M. P. M., Fauth, U., and Demain, A. L., Appl. Environ. Microbiol., 1990, 56 , 3040-3046). Avicelase specific activity of this homogeneous subpopul ation was slightly higher than that of unfractionated cellulosomes, bu t the two preparations were similarly affected by Ca2+, dithiothreitol , and cellobiose. Determination of their N-terminal sequences and enzy me activities has enabled three of the six major subunits of the subpo pulation of cellulosomes to be positively identified as known componen ts of the C. thermocellum cellulase complex; the other three subunits did nor match up with previously characterized cellulosomal proteins.