GLY387 OF MURINE ORNITHINE DECARBOXYLASE IS ESSENTIAL FOR THE FORMATION OF STABLE HOMODIMERS

In its active form mammalian ornithine decarboxylase (ODC) is a homodi mer composed of two 53-kDa subunits while the monomer retains no enzym ic activity. In the present study we demonstrate that Gly387 of mouse ODC plays an important role in enabling dimer formation. Gly387 of mou se ODC, an evolutionary conserved residue, was converted to all possib le 19 amino acids using site-directed mutagenesis. With the exception of alanine, all other substitutions of Gly387 completely abolished enz ymic activity. Cross-linking analysis and fractionation through a Supe rose-12 sizing column have demonstrated that mutant subunits are detec ted only in their monomeric form. These results strongly suggest that the primary lesion of substitution at position 387 of mouse ODC is the inability of mutant subunits to associate with each other to form the active homodimers. In agreement with this conclusion, G387 A, the onl y mutant that retained partial activity, displayed reduced dimerizatio n. The degradation rate of ODC mutants in which Gly387 was substituted by aspartic acid or alanine was enhanced compared to the wild-type en zyme, suggesting that monomers may be more susceptible to degradation.