Structural and functional organization of the Na+, K+-pump.

The active transport of Na’ and K + across cell membranes is catalysed by the ubiquitous transport system known as the Na+,K+-pump. Historically, Dean [ l] was the first to propose the name sodium pump for this physiological process, and in 1057 Skou discovered the enzyme, the Na+ ,K +-activated ATPase, soon shown to be identical to the transport system [2]. The structure and mechanism of Na+,K+-ATPase have been the subjects of study by many groups since then, and lately molecular biology and experiments on the physiological regulation of transport have added to our knowledge. Most of these aspects, including the history of active transport, are treated in the papers and overviews published as the proceedings of the fifth international conference in this field [ 31. Relevant references to other work discussed in this article can be found there [3] and in more recent extensive reviews by Glynn [4], J~rrgensen & Andersen [5] and Skou [6]. One of the unsolved problems in the field of ion transport is whether the functional system is monomeric or oligomeric [7]. The Na+,K+-pump has two different, closely associated subunits called a and /3, and some researchers are energetic proponents of the theory, that an ap protomer can perform all the catalytic functions. Others seem convinced that an ( a/B),-diprotomer, with functional associations and interactions between the catalytic subunits, is the minimum requirement for coupling of transport and ATP hydrolysis [8, 91. This article gives a short review of some of the structural and functional information needed in the attempt to elucidate the monomeric/oligomeric nature of Na+ ,K+-transport. In addition, some recent studies on ligand binding and radiation inactivation, that may throw new light on this problem, are described in brief.