Catalysis of the reaction: 2 S-adenosyl-L-methionine + a  L-aspartate89 + a sulfurated  + a reduced electron acceptor = S-adenosyl-L-homocysteine + L-methionine + 5'-deoxyadenosine + 2 H+ + a  3-methylthio-L-aspartate89 + an unsulfurated  + an oxidized electron acceptor.
protein methylthiotransferase activity