The following are steps which incompletely describe the mechanism by which Na+ and K+ ions are transported by the (Na+—K+)—ATPase. What is the correct sequence for the events listed, assuming that K+ ions have just dissociated? 1. The phosphate group on the protein is hydrolyzed. 2. ATP binds to the protein. 3. The protein's conformation changes, exposing K+ binding sites to the cell interior. 4. Na+ ions dissociate.