We study the molecular structure of P-type ion pumps with the overall aim of revealing the physical basis of ion transport. Using Ca-ATPase and Na,K-ATPase to represent a large family of ion pumps, we investigate the physical relationship between sites of ATP- and ion-binding and the nature of the conformational changes used to couple these two sites. Following a muscle contraction, Ca-ATPase uses ATP to pump calcium across the SR membrane, thereby lowering calcium concentrations inside the cell and allowing the muscle to relax. Na,K-ATPase is ubiquitous in plasma membranes of animal cells and responsible for maintaining the resting membrane potential and ionic homeostasis. For our structural studies, we have imaged tubular crystals in a frozen, unstained state with an electron microscope. We then use a wide range of computer programs to reconstruct and to display the three-dimensional structure. Thus, we have determined structures of Ca-ATPase at 6.5 A resolution and Na,K-ATPase at 10 A resolution. By fitting these with the known atomic structure of Ca-ATPase, we have deduced the large-scale structural changes that accompany several steps of the reaction cycle.

Tissue organization and strength is first established and then maintained by adhesive junctions. There are two types, termed adherens junction and desmosomes, which have analogous architectures but different molecular constituents. We are using electron tomography to study their molecular architecture. As a first step, we have determined structures of desmosomes from mouse epidermis and have revealed the organization of cadherin molecules within the intercellular space. Cadherins are responsible for providing the physical adhesion between cells and their mode of interaction has generated considerable controversy within the literature. Our structure shows for the first time their mode of interaction within an intact junction and suggests particular mechanisms for assembly and maintenance of the desmosome.