FUNCTIONAL SIZE OF C-TERMINAL PROTEIN CARBOXYL METHYLTRANSFERASE FROMKIDNEY BASOLATERAL PLASMA-MEMBRANES

The functional sizes of the C-terminal isoprenylcysteine protein carbo xyl methyltransferase (PCMT) from kidney cortex basolateral plasma mem branes and yeast membranes have been estimated by the radiation inacti vation and fragmentation method. Attempts to solubilize the methyltran sferase with detergents were unsuccessful as they resulted in the irre versible denaturation of its enzymatic activity. The radiation inactiv ation sizes of the methyltransferases were 98 and 24 kDa for kidney an d yeast, respectively. Kinetic experiments showed that irradiation aff ects the V-max of the reaction but not the apparent K-m for either S-a denosyl-L-methionine and N-acetyl farnesylcysteine. The functional siz e reported here for the kidney membrane is about 4-times larger than t he size predicted for the Saccharomyces cerevisiae C-terminal PCMT ded uced from the nucleotide sequence of its gene (28 kDa). These results suggest that mammalian methyltransferase has a functional size differe nt from that of the yeast; tetramerization of monomers is one possible hypothesis for this difference.