Interaction between the endoglucanase CelA and the scaffolding protein CipC of the Clostridium cellulolyticum cellulosome.

The 5' end of the cipC gene, coding for the N-terminal part of CipC, the scaffolding protein of Clostridium cellulolyticum ATCC 35319, was cloned and sequenced. It encodes a 586-amino-acid peptide, including several domains: a cellulose-binding domain, a hydrophilic domain, and two hydrophobic domains (cohesin domains). Sequence alignments showed that the N terminus of CipC and CbpA of C. cellulovorans ATCC 35296 have the same organization. The mini-CipC polypeptide, containing a cellulose-binding domain, hydrophilic domain 1, and cohesin domain 1, was overexpressed in Escherichia coli and purified. The interaction between endoglucanase CelA, with (CelA2) and without (CelA3) the characteristic clostridial C-terminal domain called the duplicated-segment or dockerin domain, and the mini-CipC polypeptide was monitored by two different methods: the interaction Western blotting (immunoblotting) method and binding assays with biotin-labeled protein. Among the various forms of CelA (CelA2, CelA3, and an intermediary form containing only part of the duplicated segment), only CelA2 was found to interact with cohesin domain 1 of CipC. The apparent equilibrium dissociation constant of the CelA2-mini-CipC complex was 7 x 10(-9)M, which indicates that there exists a high affinity between these two proteins.