Structure of Plant Cell Walls

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Abstract
Wild type B. subtilis, when grown on beet araban, secretes into its culture medium an endo-arabanase and 2 arabinosidases. An alternate procedure to one previously described was developed for the purification of the endo-arabanase. The purified endo-arabanase is homogeneous by sodium dodecyl sulfate-urea disc gel electrophoresis (MW .simeq. 32,000) and by isoelectric focusing (pl = 9.3). The endo-arabanase, acting on a branched araban substrate, has maximal activity at pH 6.0 and preferentially cleaves 5-linked arabinosyl residues. One of the arabinosidases (MW .simeq. 65,000, pl = 5.3) was purified to the point that it contains only 1 quantitatively minor contaminant, as shown by sodium dodecyl sulfate-urea disc gel electrophoresis and isoelectric focusing. The purified arabinosidase, acting on p-nitrophenyl-.alpha.-L-arabinofuranoside, has maximal activity at pH 6.5 and, when acting on a branched araban substrate, preferentially attacks nonreducing terminal arabinosyl residues linked to the 2 or 3 positions of other arabinosyl residues. Neither of the 2 purified enzymes is capable of hydrolyzing a variety of carbohydrate substrates which lack arabinosidic linkages. The purified endoarabinase is capable of releasing arabinosyl oligomers from the walls of suspension-cultured sycamore cells, thereby suggesting its usefulness as a probe in studying the structure of the araban component of primary cell walls.