JBC, Vol. 250, Issue 17, 6648-6652, Sep, 1975
Subunit structure of anthranilate synthetase from Neurospora crassa
F. M. Hulett and J. A. DeMoss
Freshly purified preparations of anthranilate synthetase complex from
Neurospora crassa appeared to be homogeneous on polyacrylamide disc gels
and were composed of two distinct subunits, 94,000 and 70,000 daltons,
respectively, as determined by electrophoresis on polyacrylamide gels in
the presence of sodium dodecyl sulfate. Carboxymethylation of the complex
or treatment with guanidine hydrochloride and urea before sodium dodecyl
sulfate treatment did not alter the subunit pattern. When the purified
complex was iodinated with 125I- or methylated with [14C]dimethylsulfate,
no labeled components other than the two subunits stained with Coomassie
blue were detected after electrophoresis in the presence of sodium dodecyl
sulfate. Although some purified preparations were stable, most were
unstable upon storage. Analysis of the unstable preparations on
nondenaturing and sodium dodecyl sulfate polyacrylamide disc gels revealed
that the complex in these preparations was progressively fragmented to
smaller components and subunits upon repeated freeze-thaw treatment or
prolonged incubation at or above 4 degrees. Distinct fragments were
generated ranging in size down to 25,000 daltons, and some fragments
retained some of the activities associated with the anthranilate synthetase
complex. On the basis of these and earlier studies, we conclude that
anthranilate synthetase from Neurospora crassa is composed of two distinct
subunits in an alpha2beta2 structure; one subunit is a trifunctional
peptide which contains the catalytic sites for the
phosphoribosylanthranilate isomerase and indoleglycerol phosphate
synthetase reactions, and associates with the second subunit to form
glutamine-dependent anthranilate synthetase. The smaller subunits and
components previously reported for this complex are apparently due to
protease activity present in purified preparations.