Rat guanidinoacetate methyltransferase: mutation of amino acids within a common sequence motif of mammalian methyltransferase does not affect catalytic activity but alters proteolytic susceptibility.

1. Manual alignment of amino acid sequences of mammalian S-adenosylmethionine-dependent methyltransferases of known sequence revealed the presence of 2 homologous regions. 2. The sequence of the region at the C-terminal side is unique to mammalian methyltransferases, and in guanidinoacetate methyltransferase this sequence occurs at residues 159-165. 3. Mutagenesis of 5 conserved residues in this sequence did not affect the catalytic activity but altered tryptic susceptibility at Arg20.