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+-A TPase have pinpointed five amino acid residues that are essential to C a2+ occlusion, and these residues have been assigned to different part s of a Ca2+ binding pocket with channellike structure, Three of the ho mologous Na+,K+-ATPase residues have been shown to be important for bi nding of cytoplasmic Na+ at transport sites. In addition, three of the above mentioned Ca2+-ATPase residues appear to participate in the cou ntertransport of K+, and two of the Na+,K+-ATPase residues to particip ate in the countertransport of KC, Residues involved in energy transdu cing conformational changes have also been identified by mutagenesis. In the Ca2+-ATPase, ATP hydrolysis is uncoupled from Ca2+ transport fo llowing mutation of a tyrosine residue located at the top of transmemb rane segment M5. This tyrosine, present also in the Na+,K+-ATPase, may play a critical role in closing the gate to a transmembrane channel.