Protein factor which induces conversion between Physarum ornithine decarboxylase forms in vitro.

The rapid activity modulation of ornithine decarboxylase (L-ornithine carboxy-lyase, EC 4.1.1.17) in Physarum polycephalum is closely correlated with a reversible post-translational modification of this enzyme. A factor has now been isolated from homogenates of exponentially-grown microplasmodia which catalyzes the conversion of the active enzyme form, A, into its less active, B state. Partial purification of this A-B converting factor has been achieved using DEAE-Sephacel chromatography and Ultrogel AcA-34 gel filtration. It appears to be a heat labile, acidic protein with a molecular weight of about 35 000 which binds to large macromolecules in crude fractions isolated using low ionic strength buffers. The in vitro converting reaction requires the presence of spermidine or spermine (1 mM) while putrescine is much less effective, and inorganic cations are ineffective at levels up to 5 mM. Enzyme conversion is reduced in elevated ionic strengths and in the presence of polyamine chelators such as ATP, ADP or GTP (1.0 mM). Under current assay conditions the interaction between ornithine decarboxylase and this factor is stoichiometric, yet it is not reversed even by conditions which favor dissociation of protein-protein interactions. This is the first report of the isolation of a protein factor which is involved in the interconversion of ornithine decarboxylase between its alternate enzyme states.