P-type ion pumps: structure determination may soon catch t,p with structure predictions

ATP-driven ion pumps fall into three families, which are known as F-type (named after F1/F 0 ATPase), V-type (named after the vacuolar H + pump), and P-type (those forming a covalent phosphoenzyme). This review focuses on P-type ion pumps, a family that shares both amino acid homologue and a basic reaction mechanism. The family includes a wide range of species (including bacteria, yeast and mammals) and displays an extraordinary diversity in ion specificity and stoichiometry. The best known members of the family are Ca 2+ATPase from the sarcoplasmic reticulum and Na+/K +ATPase from plasma membranes, both of which have a long history of investigation. Besides detailed studies of the reaction cycle, many techniques have been used to link features of this cycle to molecular structure, e.g. effects of site-specific labels, fluorescence both of labels and of intrinsic tryptophan residues, infrared spectroscopy, proteolysis, crosslinking and antibody binding. An important template for considering the resultant flood of biophysical data has been obtained more recently, from the cloning and deduction of amino acid sequences for several P-type pumps. These sequences have been used to predict secondary structure and, when considered together with various biophysical data, have led to a detailed model for a consensus structure of the molecule (Fig. 1). The roles of individual amino acids have recently been investigated in studies on a large number of site-directed mutations of Ca 2 +ATPase and their effects on fundamental assays such as Ca 2 + transport and phosphorylation. Thus, phosphorylation and ATP-binding sites have been confirmed, several residues in the center of the transmembrane domain have been implicated in Ca 2 + binding and residues involved in the transduction of energy between these sites have been identified. Direct structural studies: electron microscopy and X-ray diffraction