Thermostable endoglucanase that has high activity with soluble polymeric substrates containing beta-1,4-glycosidic bonds, such as carboxymethyl cellulose (CMC) and barley beta-D-glucan (in vitro). Has no activity with cellobiose and filter paper. Has no activity with substrates containing beta-1,3-linked glycans, such as laminarin. Likewise, lacks activity with xylan, galactomannan and pectin. Endohydrolysis of (1->4)-beta-D-glucosidic linkages in cellulose, lichenin and cereal beta-D-glucans. Activity is enhanced by 1mM Mn(2+), but is not affected by 1mM Ca(2+), Mg(2+), Zn(2+), K(+), Na(+) or Li(+). Activity is not inhibited by EDTA (in vitro). kcat is 46.3 sec(-1) with carboxymethyl cellulose as substrate. Optimum pH is 5 with carboxymethyl cellulose as substrate. Retains 80% activity at pH 6, and about 50% at pH 7. Inactive at pH values below 4 and above 9. Optimum temperature is 70 degrees Celsius. Retains about 85% activity after 24 hours at 65 degrees Celsius. Retains at least 50% activity after 4 hours at 70 degrees Celsius. Rapidly looses activity at 75 degrees Celsius. Monomer. Thermostable endoglucanase that retains activity in the presence of high salt concentrations, suggesting it might be used as an industrial enzyme for cellulose-based substrates. Belongs to the glycosyl hydrolase 5 (cellulase A) family.