Two cellulases, CelA and CelC, from the polycentric anaerobic fungus Orpinomyces strain PC-2 contain N-terminal docking domains for a cellulase-hemicellulase complex

Two cDNAs encoding two cellulases, CelA and CelC, were isolated from a cDNA library of the polycentric anaerobic fungus Orpinomyces sp. strain PC-2 constructed in Escherichia coli. Nucleotide sequencing revealed that the celA cDNA (1,558 bp) and celC cDNA (1,628 bp) had open reading frames encoding polypeptides of 459 (CelA) and 449 (CelC) amino acids, respectively. The two cDNAs were 76.9 and 67.7% identical at the nucleotide and amino acid levels, respectively. Analysis of the deduced amino acid sequences showed that starting from the N termini, both CelA and CelC had signal peptides, which were followed by noncatalytic repeated peptide domains (NCRPD) containing two repeated sequences of 33 to 40 amino acid residues functioning as docking domains. The NCRPDs and the catalytic domains were separated by linker sequences. The NCRPDs were homologous to those found in several hydrolases of anaerobic fungi, whereas the catalytic domains were homologous to the catalytic domains of fungal cellobiohydrolases and bacterial endoglucanases. The linker sequence of CelA contained predominantly glutamine and proline residues, while that of CelC contained mainly threonine residues. CelA and CelC did not have a typical cellulose binding domain (CBD). CelA and CelC expressed in E. coli rapidly decreased the viscosity of carboxymethyl cellulose (CMC), indicating that there was endoglucanase activity. In addition, they produced cellobiose from CMC, acid-swollen cellulose, and cellotetraose, suggesting that they had cellobiohydrolase activity. The optimal activity conditions with CMC as the substrate were pH 4.3 to 6.8 and 50 degrees C for CelA and pH 4.6 to 7.0 and 40 degrees C for CelC. Despite the lack of a CBD, CelC displayed a high affinity for microcrystalline cellulose, whereas CelA did not.