ABSTRACT Many of the bilayer spanning segments of membrane transport proteins con-tain proline residues, and most of them are be-lieved to occur in a-helical form. A proline res-idue in the middle of an a-helix is known to produce a bend in the helix, and recent studies have focused on characterizing such a bend at Apart from its presence in globular proteins, the proline residue in the middle of the membrane span-ning segments of a number of transport proteins has been identified.7 The secondary structure of such membrane spanning segments has been character-ized as a-helical in bacteriorhodopsin (BR) ' and other membrane proteins are also known to have atomic level. In the present case, molecular