Co-purification from Escherichia coli of a plant β-glucosidase-glutathione S-transferase fusion protein and the bacterial chaperonin GroEL

The coding sequence of the mature cyanogenic β-D-glucosidase (β-D-glucoside glucohydrolase, EC 3.2.1.21) (linamarase) of Manihot esculenta Crantz (cassava) was cloned into the vector pGEX-2T and expressed in Escherichia coli. The bacterial chaperonin GroEL [Braig, Otwinowski, Hedge, Boisvert, Joachimiak, Horwich and Sigler (1994) Nature (London) 371, 578–586] was found to be tightly associated with the fusion protein and co-purified with it. In the presence of excess MgATP, release and folding of the fusion β-glucosidase were demonstrated by a fast increase in both linamarase and p-nitrophenyl-β-D-glucopyranosidase activity at a low protein concentration. A slow endogenous folding process was also detected by activity measurements. Michaelis constants (K_m) and the ratio between the maximal velocities and efficiency constants (V_max., V_max./K_m) for the hydrolysis of the natural substrate, linamarin, and p-nitrophenyl β-D-glucopyranoside (PNP-Glc) by the recombinant protein were found to be almost identical with those of the native glycosylated plant enzyme [Keresztessy, Kiss and Hughes (1994) Arch. Biochem. Biophys. 314, 142–152]. Molecular dissociation constants for the free enzyme (pK^E₁, pK^E₂) obtained with linamarin and PNP-Glc, and the enzyme substrate complexes (pK^ES₁, pK^ES₂) were also in accordance with that of the original protein. The reactive substrate analogue N-bromoacetyl β-D-glucosylamine inactivated the fusion enzyme according to pseudo-first-order kinetics with first-order rate constant (k_i = 0.007 min^-1) and apparent inhibition constants (K_i = 20 mM) comparable with those of the plant protein [Keresztessy, Kiss and Hughes (1994) Arch. Biochem. Biophys. 315, 323–330]. In comparison with the native glycosylated plant protein, the recombinant protein was, however, found to be extremely sensitive to proteolysis and misfolding.