[Rotational mobility of membrane-bound Na,K-ATPase].

The rotational mobility of E1 and E2 conformers of duck salt gland Na,K-ATPase labelled with eosine-5'-isothiocyanate (EITC) was studied using a time-resolved phosphorescence anisotropy approach. For each conformer, two types of the rotational mobility were found. The rotational correlation time of the faster component equal to about 15 microseconds at 20 degrees for the both conformers, was ascribed to the rotation of the (alpha beta) protomer with an apparent radius 2.4 nm. The slower component (100-500 microseconds depending on experimental conditions) was suggested to reflect the presence in the bilayer of associates between Na,K-ATPase molecules or those with other protein constituents of the membrane bilayer. A rise in temperature tends to decrease the fast component with a subsequent increase in the slow component of the experimental curve, apparently due to oligomerisation of the protomers into oligomers. The size of the oligomers depends on pH and temperature and under favourable conditions may come up to octamers.