Biophysical Journal 60: 1315-1325 (1991)
© 1991 the Biophysical Society

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Intermediate structures in the cholate-phosphatidylcholine vesicle-micelle transition

Anne Walter ^*, Phillip K. Vinson , Alon Kaplun  and Yeshayahu Talmon

Department of Physiology and Biophysics, Wright State University, Dayton, Ohio 45435
Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455
Department of Chemical Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel

ABSTRACT

The vesicle-micelle transition of egg phosphatidylcholine (PC) and sodium cholate was described by comparing cryo-transmission electron microscopic (cryo-TEM) images of the structures formed to the associated turbidity changes. These experiments were designed to identify the morphology of the intermediates between vesicles and small spheroidal mixed micelles. With increasing cholate concentration, the vesicular structures changed size and more multilamellar vesicles were seen. Between the apparent upper and lower phase boundaries, three structures were observed: open vesicles, large bilayer sheets (twenty to several hundred nanometers in diameter), and long (150-300 nm) flexible cylindrical micelles. The cylindrical micelles evolved from the edges of the bilayer sheets. At higher relative cholate concentration, the phase boundary was sharply defined by optical clarification of the egg PC-cholate mixtures. Cryo-TEM revealed only small spheroidal mixed micelles at this transition. These results provide the first direct evidence of the structural pathway or of molecular intermediates between a lamellar and a micellar state. Understanding these specific intermediates and the transitions between them is essential to developing reconstitution protocols and properly analyzing either activity or structural data obtained from cholate-dispersed membrane proteins.