Converts guanidinoacetate to creatine, using S-adenosylmethionine as the methyl donor. Important in nervous system development. guanidinoacetate + S-adenosyl-L-methionine = creatine + H(+) + S-adenosyl-L-homocysteine Amine and polyamine biosynthesis; creatine biosynthesis; creatine from L-arginine and glycine: step 2/2. Monomer. May form homodimers upon proteolytic removal of the first 36 amino acid residues. Expressed in hepatic primordium in the embryo as soon as 12.5 days. In the adult, high levels of expression are found in liver and pancreas. Ubiquitously expressed in neuronal and glial cells in the brain. The N-terminal first 36 amino acid residues are susceptible to proteolytic cleavage, leading to loss of activity. Belongs to the class I-like SAM-binding methyltransferase superfamily. RMT2 methyltransferase family.