Inhibition of erythrocyte membrane shape change by band 3 cytoplasmic fragment

The ATP-dependent transformation of crenated white human erythrocyte ghosts into smoothed disc and cup forms is inhibited by the soluble 40-45-kilodalton (kDa) cytoplasmic portion of the major transmembrane protein, band 3. The band 3 fragment was prepared by chymotryptic treatment of inverted vesicles stripped of peripheral proteins. When present at greater than or equal to 0.2 mg per mg membrane protein (ie, greater than or equal to 2 mol fragment per mol endogenous band 3), the fragment significantly reduced the rate of shape change but did not alter the proportion of membranes that were ultimately converted into smoothed forms (greater than 90%). The inhibitory activity of the fragment could not be attributed to contamination of the fragment preparation by actin or proteolytic enzymes. ATP-independent shape transformation was not inhibited. The band 3 fragment may compete with endogenous, intact band 3 for an association with the spectrin-actin network required for ATP-dependent smoothing of crenated membranes.