Abstracts Biological Sciences

Tissue-specific regulation of the sodium potassium adenosine triphosphatase

by Alex Geoffroy. Therien

The Na,K-ATPase, or sodium pump, is a membrane-associated protein complex comprising two subunits, alpha and beta, both of which can exist as one of several isoforms. It generates and maintains the electrochemical Na + and K+ gradients across the plasma membrane of animal cells. These gradients are the driving force for a variety of ubiquitous and specialized cell functions, such as transport of solutes, as well as maintenance of membrane potential and cell volume. Agents that modulate the kinetic behaviour of the pump enable cells to adapt to changing needs. Distinct substrate activation profiles in various tissues presumably underlie the specialized functions of the sodium pump in these tissues. Although the tissue-specific distribution of various isoforms accounts for some of the differences in kinetic behaviour of the enzyme, other factors are also important determinants of such behaviour. This study describes the characterization of two mechanisms of sodium pump modulation. The first involves alterations in the susceptibility of the enzyme to competitive inhibition by K+ at Na+ binding sites. Studies on the alpha1 and alpha3 isoform of various tissues and cells have revealed that there exist tissue-specific components that determine the extent to which the two cations compete with each other for cytoplasmic binding sites. Specifically, pumps that are highly susceptible to K +/Na+ antagonism also bind and occlude K+ much more readily on the cytoplasmic site, and this behaviour is abrogated upon fusion of pumps into another membrane environment, that of the red blood cell. The second mechanism of regulation of pump behaviour described in this thesis involves modulation of the apparent affinity of the enzyme for ATP by the gamma subunit. This membrane protein had been previously cloned and sequenced, but very little was known about its function. This study shows that the gamma subunit is expressed uniquely in kidney tubules, and has a C-terminus-in, N-terminus-