Production and composition of an exocellular Nigeran-protein complex isolated from cultures of Aspergillus awamori.

Aspergillus awamori Nakazawa (QM873) hyphae maintained on a nitrogen-deficient medium produced an exocellular complex consisting of the wall alpha-glucan, nigeran (94%), water-soluble nigeran oligosacchrides (1 to 2%), protein (2 to 4%), and a small amount of beta-glycan (less than 1%). Nigeran was not covalently linked to protein. The complex appeared in the growth medium only under those temporal or nutritional conditions where the hyphal wall content to nigeran reached at least 30% of the cell dry weight. Rates of nigeran accumulation in the hypha cell wall, scanning electron microscopy of hyphae, and pulse-chase experiments with [14C]glucose suggested that the complex arises via displacement of a portion of the hyphal wall into the medium. The nigeran portion of the complex contained lamellar crystalline domains similar to those in the intact cell wall. Enzymic digestion of nigeran in the complex indicated that it has a degree of crystallinity lower than that observed with pure nigeran crystals grown in vitro. Therefore, polymerization in vivo resulted in somewhat less chain organization in the crystallite. Since this complex was isolated without any prior chemical or exogenous enzymatic treatment, it should be useful for studies of hyphal wall biogenesis and organization in this organism.

Production by Aspergillus awamori of homologous oligosaccharides, each with an uneven number of glucosyl units and a reducing terminal (alpha , 1 leads to 4) linkage.

Homologous oligosaccharides with degrees of polymerization of 7 to 15 have been isolated from the nigeran-protein complex of Aspergillus awamori Nakazawa (QM873). Each contains an uneven number of glucose units, differs from its nearest homolog by two residues, and contains the sequence Glc(alpha , 1 leads to 3)Glc- (alpha , 1 leads to 4)Glc at its reducing terminus.

Distribution and conformation of crystalline nigeran in hyphal walls of Aspergillus niger and Aspergillus awamori.

Hyphal walls of Aspergillus awamori containing increased amount of the alpha-glucan, nigeran, became correspondingly more opaque when viewed in the electron microscope as shadowed preparations. However, increased polymer deposition was not accompanied by any significant change in wall thickness. The nigeran of both A. awamori and Aspergillus niger occurred in situ in a crystalline conformation identical to that of single crystals prepared with pure polysaccharide. Furthermore, this polymer was the dominant crystalline material in the hyphae whether or not they were enriched in nigeran. Enzymic digestion of nigeran in A. niger and A. awamori revealed that the bulk of the polymer was exposed to the cell's exterior. However, a certain fraction was accessible to enzymic attack only after the wall was treated with boiling water. A third portion, detectable only by x-ray diffraction, was associated with other components and could not be extracted, even with prolonged boiling. It was removed by hot, dilute alkali and was associated in the wall with another glucan fraction. Dry heating of A. niger walls altered their susceptibility to enzymic digestion of nigeran in situ. It is proposed that this treatment introduces interstices in the crystal surface that facilitate attack.