Inhibition of myosin ATPase activity by human myasthenia gravis antibodies reactive with the acetylcholine receptor

Antibodies, obtained from myasthenia gravis patients, with reactivity for an immunodominant region of the nieotinic acetylcholine receptor were found to also react with muscle myosin. Since amino acid sequence analyses have previously suggested possible serological relationships between AChR and a head region sequence of myosin heavy chain, cross-reactive antibodies were examined for their ability to interfere with ATPase activities associated with this region of myosin. Results indicated that AChR-specific antibodies purified from MG patient serum by binding to and elution from antigen columns were found to inhibit Ca²⁺-dependent, myosin-associated ATPase activity; interestingly, this inhibition appeared to be relatively selective in that neither K⁺(EDTA)-dependent nor Mg²⁺-dependent ATPase activities were sensitive to antibody-mediated interference.