Three forms of cellobiohydrolase I from Trichoderma reesei.

Three forms of cellobiohydrolase I (CBH I) (65, 58 and 54 kDa) were isolated to apparent homogeneity from culture filtrates of Trichoderma reesei. The N-terminal sequence of amino acid residues is the same for all of them. The 65 kDa CBH I (pI 4.1) is the intact protein which is fully active against small, soluble substrates and an insoluble substrate Avicel. The 58 kDa CBH I (pI 3.8) and 54 kDa CBH I (pI 3.6) are two truncated forms of the intact CBH I, which are fully active against small, soluble substrates, but have decreased adsorption on and activity against Avicel. Limited proteolysis of the 65 kDa and 58 kDa CBH I by papain yields the same core protein (pI 3.6, 54 kDa). It appears that there are mainly two different specific proteolytic cleavage points in the intact CBH I, one the site for a papain-like protease action cutting at the hinge area (54 kDa CBH I) and the other in the B block (58 kDa CBH I).