Characterization of subspecies from a fungal fatty acid synthetase

Fatty acid synthetase from the filamentous fungus, Aspergillus fumigatus dissociates during gel filtration on Sepharose 6B into two differently sized subspecies with mol. wt. approx. 1.5 x 10(6) and 8 x 10(5). After elution, they readily reform intact molecules, as determined by their enzymic activity (overall synthetase and 3-oxoreductase activities were measured), sedimentation coefficient and appearance in the electron microscope. Synthetase was cross-linked with dimethyl suberimidate and the resultant protein did not dissociate on Sepharose 6B. The two smaller species which were eluted after chromatography of untreated enzyme were also fixed by reaction with this reagent. They did not reform intact molecules of synthetase and were characterized by electron microscopy as large and small circular aggregates; the low molecular weight form also contained tetrameric structures which exhibited cyclic symmetry. The composition of the two species derived during dissociation was, therefore, confirmed as eight and four polypeptides, respectively; each contained polypeptides A and B. It is proposed that the intact fungal synthetase of composition A6B6 comprises three equivalent loops of protein, each of which contain four polypeptides, presumably with composition A2B2; the molecular weights of A and B are 207 000 and 201 000, respectively. During filtration on Sepharose 6B, two such loops remain associated to form a large circle, leaving the other four polypeptides ro rearrange themselves into a small circle or tetramer.