non-catalytic cellulose-binding domain of a novel cellulase from Pseudomonas fluorescens subsp. cellulosa is important for the efficient hydrolysis of Avicel

A genomic library of Pseudomonas fluorescens subsp. cellulosa DNA, constructed in lambdaZAPII, was screened for carboxymethyl-cellulase activity. The pseudomonad insert from a recombinant phage which displayed elevated cellulase activity in comparison with other cellulase-positive clones present in the library, was excised into pBluescript SK- to generate the plasmid pC48. The nucleotide sequence of the cellulase gene, designated celE, revealed a single open reading frame of 1710 bp that encoded a polypeptide, defined as endoglucanas E (CelE), of Mr 59663. The deduced primary structure of CelE revealed an N-terminal signal peptide followed by a 300-amino-acid sequence that exhibited significant identity with the catalytic domains of cellulase belonging to glycosyl hydrolase Family 5. Adjacent to the catalytic domain was a 40-residue region that exhibited strong sequence identity to non-catalytic domains located in two other endoglucanass and a xylanase from P. fluorescens. The C-terminal 100 residues of CelE were similar to Type-I cellulose binding domains (CBDs). The three domains of the cellulase were joined by linker sequences rich in serine residues. Analysis of the biochemical properties of full-length and truncated derivatives of CelE confirmed that the enzyme comprised an N-terminal catalytic domain and a C-terminal CBD. Analysis of purified CelE revealed that the enzyme had an Mr of 56000 and an experimentally determined N-terminal sequence identical to residues 40-54 of the deduced primary structure of full-length CelE. The enzyme exhibited an endo mode of action in hydrolysing a range of cellulosic substrates including Avicel and acid-swollen cellulose, but did not attack xylan or any other hemicelluloses. A truncated form of the enzyme, which lacked the C-terminal CBD, displayed the same activity as full-length CelE against soluble cellulose and acid-swollen cellulose, but exhibited substantially lower activity than the full-length cellulase against Avicel. The significance of these data in relation to the role of the CBD is discussd.