Functional size of C-terminal protein carboxyl methyltransferase from kidney basolateral plasma membranes

The functional sizes of the C-terminal isoprenylcysteine protein carboxyl methyltransferase (PCMT) from kidney cortex basolateral plasma membranes and yeast membranes have been estimated by the radiation inactivation and fragmentation method. Attempts to solubilize the methyltransferase with detergents were unsuccessful as they resulted in the irreversible denaturation of its enzymatic activity. The radiation inactivation sizes of the methyltransferases were 98 and 24 kDa for kidney and yeast, respectively. Kinetic experiments showed that irradiation affects the Vmax of the reaction but not the apparent Km for either S-adenosyl-L-methionine and N-acetyl farnesylcysteine. The functional size reported here for the kidney membrane is about 4-times larger than the size predicted for the Saccharomyces cerevisiae C-terminal PCMT deduced from the nucleotide sequence of its gene (28 kDa). These results suggest that mammalian methyltransferase has a functional size different from that of the yeast; tetramerization of monomers is one possible hypothesis for this difference.