Ultrastructural studies of the interaction of spectrin with
phosphatidylserine liposomes
AM Cohen, SC Liu, LH Derick and J Palek
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
rotary-shadowed platinum replicas of spectrin dimer-phosphatidylserine
complexes to study the interaction of spectrin with phosphatidylserine
vesicles. At a spectrin concentration of 0.6 mg/mL, 60% of spectrin dimers
were associated with phosphatidylserine vesicles and at a spectrin
concentration of 1.2 mg/mL, 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 degrees and the contact region to
phosphatidylserine extended 272 +/- 144A degrees 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 phosphatidylserine liposomes, extended 1804 +/-
79A degrees from the liposome edge and crosslinked liposomes, suggesting
that some of the binding sites to phosphatidylserine vesicles is in the
proximity of the tail end of spectrin. The association between spectrin
dimers to phosphatidylserine was demonstrated by nondenaturing gel
electrophoresis. The complexes were separated into multiple bands with
molecular weight of 1.4 X 10(6), 1.8 X 10(6), and 2.3 X 10(6). These bands
did not represent self- associated spectrin oligomers, since postincubation
treatment with Triton-X-100 dissociated them into spectrin dimers.
Furthermore, these spectrin high molecular weight bands, as visualized by
Coomassie blue absorbance, closely corresponded to the
14C-phosphatidylserine distribution. These data provide ultrastructural and
biochemical evidence that spectrin binds to phosphatidylserine at multiple
sites including the tail end region.
Volume 68,
Issue 4,
pp. 920-926,
10/01/1986
Copyright © 1986 by The American Society of Hematology
