Reduced Spectrin-Ankyrin Binding in a South African Hereditary Elliptocytosis Kindred Homozygous for Spectrin St Claude

Control of erythrocyte membrane-skeletal cohesion by the spectrin-membrane linkage

Spectrin tetramer is the major structural member of the membrane-associated skeletal network of red cells. We show here that disruption of the spectrin-ankyrin-band 3 link to the membrane leads to dissociation of a large proportion of the tetramers into dimers. Noncovalent perturbation of the linkage was induced by a peptide containing the ankyrin-binding site of the spectrin beta-chain, and covalent perturbation by treatment with the thiol reagent, N-ethylmaleimide (NEM). This reagent left the intrinsic self-association capacity of the spectrin dimers unaffected and disturbed only the ankyrin-band 3 interaction. The dissociation of spectrin tetramers on the membrane into functional dimers was confirmed by the binding of a spectrin peptide directed against the self-association sites. Dissociation of the tetramers resulted, we infer, from detachment of the proximal ends of the constituent dimers from the membrane, thereby reducing their proximity to one another and thus weakening their association. The measured affinity of the interaction of the peptides with the free dimer ends on the membrane permits an estimate of the equilibrium between intact and dissociated tetramers on the native membrane. This indicates that in the physiological state the equilibrium proportion of the dissociated tetramers may be as high as 5-10%. These findings enabled us to identify an additional important functional role for the spectrin-ankyrin-band 3 link in regulating spectrin self-association in the red cell membrane.

Hereditary elliptocytosis: spectrin and protein 4.1R

Hereditary elliptocytosis (HE) is a common disorder of erythrocyte shape, occurring especially in individuals of African and Mediterranean ancestry, presumably because elliptocytes confer some resistance to malaria. The principle lesion in HE is mechanical weakness or fragility of the erythrocyte membrane skeleton due to defects in alpha-spectrin, beta-spectrin, or protein 4.1. Numerous mutations have been described in the genes encoding these proteins, including point mutations, gene deletions and insertions, and mRNA processing defects. Several mutations have been identified in a number of individuals on the same genetic background, suggesting a "founder effect." The majority of HE patients are asymptomatic, but some may experience hemolytic anemia, splenomegaly, and intermittent jaundice.

Spectrin oligomerization is cooperatively coupled to membrane assembly: a linkage targeted by many hereditary hemolytic anemias?

In the erythrocyte, ankyrin is the major adapter protein linking tetramers of band 3 to the spectrin-actin cytoskeleton. This linkage involves a direct interaction between ankyrin and the 14th-15th repeat unit of beta-spectrin. The spectrin cytoskeleton itself is stabilized by the self-association of spectrin heterodimers into tetramers and larger oligomers, a process mediated by the 17th repeat unit of beta-spectrin and a short NH(2) -terminal sequence in alpha-spectrin. The self-association of spectrin and its ankyrin-mediated membrane binding have generally been considered independent events. We now demonstrate that spectrin self-association, the binding of spectrin to ankyrin, and the binding of ankyrin to the 43-kDa cytoplasmic domain of band 3 (cdb3) are coupled in a positively cooperative way. In solution, [(125)I]-labeled ankyrin was found by ND-PAGE3 to enhance the affinity of spectrin self-association by 10-fold. The reciprocal process was also true, in that spectrin tetramers and oligomers bound ankyrin with enhanced affinity relative to dimer spectrin. Saturation of the beta-spectrin self-association site by an NH(2) -terminal 80-kDa alpha-spectrin peptide enhanced the affinity of spectrin dimer for ankyrin, indicating a direct relationship between ankyrin binding and the occupancy of the beta-spectrin self-association site. cdb3 accentuated these cooperative interactions. Several inherited spectrin mutations that cause hemolytic disease but that do not directly destabilize the self-association or ankyrin-binding sites can be explained by these results. Three classes of mutations appear to disrupt cooperative coupling between self-association and ankyrin binding: (i) mutation of the linker sequences that join helices C and A in repeat units that intervene between the two functional sites, mutations that presumably block repeat-to-repeat transfer of conformational information; (ii) mutations in alpha-spectrin repeats 4 to 6 that disrupt the ability of this region to trans-regulate ankyrin binding by the adjacent beta-spectrin repeats 14-15; and (iii) exon-skipping mutations that shorten alpha-spectrin and force repeats 4 to 6 to fall out-of-register with the ankyrin-binding motif in beta-spectrin. Collectively, these results demonstrate a molecular mechanism whereby a membrane receptor can directly promote cytoskeletal assembly.