STRUCTURE-FUNCTION-RELATIONSHIPS OF CATION TRANSLOCATION BY CA2+-ATPASE AND NA+,K+-ATPASE STUDIED BY SITE-DIRECTED MUTAGENESIS

Site-directed mutagenesis studies of the sarcoplasmic reticulum Ca2+-ATPase have pinpointed five amino acid residues that are essential to Ca2+ occlusion, and these residues have been assigned to different parts of a Ca2+ binding pocket with channellike structure, Three of the homologous Na+,K+-ATPase residues have been shown to be important for binding of cytoplasmic Na+ at transport sites. In addition, three of the above mentioned Ca2+-ATPase residues appear to participate in the countertransport of K+, and two of the Na+,K+-ATPase residues to participate in the countertransport of KC, Residues involved in energy transducing conformational changes have also been identified by mutagenesis. In the Ca2+-ATPase, ATP hydrolysis is uncoupled from Ca2+ transport following mutation of a tyrosine residue located at the top of transmembrane segment M5. This tyrosine, present also in the Na+,K+-ATPase, may play a critical role in closing the gate to a transmembrane channel.