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Important residue (G46) in erythroid spectrin tetramer formation

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Abstract

Spectrin tetramerization is important for the erythrocyte to maintain its unique shape, elasticity and deformability. We used recombinant model proteins to show the importance of one residue (G46) in the erythroid α-spectrin junction region that affects spectrin tetramer formation. The G46 residue in the erythroid spectrin N-terminal junction region is the only residue that differs from that in non-erythroid spectrin. The corresponding residue is R37. We believe that this difference may be, at least in part, responsible for the 15-fold difference in the equilibrium constants of erythroid and non-erythroid tetramer formation. In this study, we replaced the Gly residue with Ala, Arg or Glu residues in an erythroid α-spectrin model protein to give G46A, G46R or G46E, respectively. We found that their association affinities with a β-spectrin model protein were quite different from each other. G46R exhibited a 10-fold increase and G46E exhibited a 16-fold decrease, whereas G46A showed little difference, when compared with the wild type. The thermal and urea denaturation experiments showed insignificant structural change in G46R. Thus, the differences in affinity were due to differences in local, specific interactions, rather than conformational differences in these variants. An intra-helical salt bridge in G46R may stabilize the partial domain single helix in α-spectrin, Helix C’, to allow a more stable helical bundling in the αβ complex in spectrin tetramers. These results not only showed the importance of residue G46 in erythroid α-spectrin, but also provided insights toward the differences in association affinity between erythroid and non-erythroid spectrin to form spectrin tetramers.

Abbreviations

αI-N1:

αI-spectrin fragment of residues 1–156

βI-C1:

βI-spectrin fragment of residues 1898–2083

G46A:

αI-N1 variant with the G46 residue replaced by Ala

G46E:

αI-N1 variant with the G46 residue replaced by Glu

G46R:

αI-N1 variant with the G46 residue replaced by Arg

ITC:

isothermal titration calorimetry

PBS:

5 mM phosphate buffer at pH 7.4 with 150 mM sodium chloride

SpαI:

erythroid α-spectrin

SpβI:

erythroid β-spectrin

SpαII:

non-erythroid α-spectrin

Tm :

temperature with 50% thermal unfolding

Umid :

urea concentration with 50% unfolding

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Correspondence to Leslie W.-M. Fung.

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Key words

  • Erythroid spectrin
  • Tetramerization
  • G46
  • mutation
  • ITC