N-terminal N-methyl-L-seryl-L-prolyl-L-lysyl-[protein] + S-adenosyl-L-methionine => N-terminal N,N-dimethyl-L-seryl-L-prolyl-L-lysyl-[protein] + S-adenosyl-L-homocysteine + H(+)
n-terminal n-methyl-l-seryl-l-prolyl-l-lysyl-[protein] + s-adenosyl-l-methionine => n-terminal n,n-dimethyl-l-seryl-l-prolyl-l-lysyl-[protein] + s-adenosyl-l-homocysteine + h(+)