label: N-terminal L-seryl-L-prolyl-L-lysyl-[protein] + 3 S-adenosyl-L-methionine => N-terminal N,N,N-trimethyl-L-seryl-L-prolyl-L-lysyl-[protein] + 3 S-adenosyl-L-homocysteine + 3 H(+)
label_lc: n-terminal l-seryl-l-prolyl-l-lysyl-[protein] + 3 s-adenosyl-l-methionine => n-terminal n,n,n-trimethyl-l-seryl-l-prolyl-l-lysyl-[protein] + 3 s-adenosyl-l-homocysteine + 3 h(+)