Sodium ions protect a membrane transport protein from proteolysis

Na+-dependent alanine transport activity in vesicles prepared from pigeon erythrocyte membranes was examined after exposure of the vesicles to some proteinases under various conditions. The presence of sodium ions during proteolysis affords considerable protection of alanine transport activity from the inhibitory action of the proteinases. The concentration of sodium ions required for half-maximum protection is greater than that needed for half-maximum activation of alanine uptake. The site of protective action could be at either or both surfaces of the membrane because the vesicles are very permeable to sodium ions. Neither measurement of residual protein content nor analysis by polyacrylamide gel electrophoresis revealed any differences in the extent of protein degradation occurring in the presence and absence of sodium ions, suggesting that the transporter constitutes only a minor membrane component. We conclude that sodium ions probably induce a conformation change in the transporter.