Ultrastructural Studies of the Interaction of Spectrin

Spectrin was shown previously to interact with phosphatidylserine and phosphatidylethanolamine. which are preferentially localized in the inner half of the membrane lipid bilayer. but this interaction is not well characterized. In the present study we used electron microscopy of rotaryshadowed platinum replicas of spectrin dimer-phosphatidylserine complexes to study the interaction of spectrin with phosphatidylserine vesicles. At a spectrin concentration of 0.6 mg I ml. 60% of spectrin dimers were associated with phosphatidylserine vesicles and at a spectrin concentration of 1 .2 mg/mI. some vesicles were crosslinked by spectrin dimers. The length of the protruding segment of spectrin dimer from the liposome edge ranged from 400 to 960A’ and the contact region to phosphatidylserine extended 272 ± 1 44A’ from either end of the molecule. Therefore. these data are consistent with multiple binding sites to phosphatidylserine throughout the spectrin dimer molecule. Spectrin tetramers. when bound to phospha-