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. Detected in microvilli of the epithelial cells lining the caput epididymis. Highly expressed in testis, caput epididymis, ovary, and liver. In the testis, localized primarily in Sertoli cells. Expressed in brain with high levels in oligodendrocytes and olfactory ensheathing glia. Moderate levels of expression in astrocytes. Belongs to the class I-like SAM-binding methyltransferase superfamily. RMT2 methyltransferase family.