STUDY OF ROTATIONAL MOBILITY OF MEMBRANE-BOUND NA,K-ATPASE

The rotational mobility of the E(1) and E(2) conformers of duck salt g land Na,K-ATPase labelled with eosine-5'-isothiocyanate (EITC) was stu died by measuring time-resolved phosphorescence anisotropy. Two types of rotational mobility were found for each conformer. The faster compo nent with rotational correlation time of about 15 mu sec at 20 degrees C for both enzyme conformers is ascribed to the rotation of the (alph a beta)-protomer on the basis of the calculated apparent rotation radi us. The slow component, whose rotational correlation time was 100-500 mu sec depending on experimental conditions, is suggested to reflect t he presence of membrane associates formed by Na,K-ATPase protomers wit h each other or with other protein constituents. Increasing temperatur e tends to decrease the fast and increase the slower component of the phosphorescence anisotropy curve, which can be explained by associatio n of protomers into oligomers of higher molecular mass. Their size dep ends on temperature and pH and may correspond to octamers under the mo st favorable conditions.