Cell wall and membrane-associated exo-beta-D-glucanases from developing maize seedlings

A beta-D-glucan exohydrolase was purified from the cell walls of developing maize (Zea mays L:) shoots. The cell wall enzyme preferentially hydrolyzes the non-reducing terminal glucosyl residue from (1-->3)-beta-D-glucans, bu t also hydrolyzes (1-->2)-, (1-->6)-, and (1-->4)-beta-D-glucosyl units in decreasing order of activity. Polyclonal antisera raised against the purifi ed exo-P-D-glucanase (ExGase) were used to select partial-length cDNA clone s, and the complete sequence of 622 amino acid residues was deduced from th e nucleotide sequences of the cDNA and a full-length genomic clone. Norther n gel-blot analysis revealed what appeared to be a single transcript, but t hree distinct polypeptides were detected in immunogel-blot analyses of the ExGases extracted from growing coleoptiles. Two polypeptides appear in the cell wall where one polypeptide is constitutive, and the second appears at the time of the maximum rate of elongation and reaches peak activity after elongation has ceased. The appearance of the second polypeptide coincides w ith the disappearance of the mixed-linkage (1-->3),(1-->4)-beta-D-glucan, w hose accumulation is associated with cell elongation in grasses. The third polypeptide of the ExGase is an extrinsic protein associated with the exter ior surface of the plasma membrane. Although the activity of the membrane-a ssociated ExGase is highest against (1-->3)-beta-D-glucans, the activity ag ainst (1-->4)-beta-D-glucan linkages is severely attenuated and, therefore, the enzyme is unlikely to be involved with turnover of the (1-->3),(1-->4) -beta-D-glucan. We propose three potential functions for this novel ExGase at the membrane-wall interface.