title: Self-association of spectrin's repeating segments.
abstract: We have examined the self-association behavior in solution of one of the repeating conformational segments of Drosophila spectrin, D-alpha-14, as well as of the two-segment unit, D-alpha-14,15. In both polypeptides, sedimentation equilibrium and nondenaturing gel electrophoresis detect a reversible, moderate affinity (K2 approximately equal to 10(4) M-1) dimerization reaction. Equilibration between monomer and dimer is kinetically limited near 5 degrees C, but occurs at a measurable rate at temperatures > or = 20 degrees C. The temperature dependence for equilibration is consistent with the requirement for extensive disruption of helix-helix packing as the reaction proceeds in either direction. Hydrodynamic studies by means of sedimentation velocity confirm that in solution the C helix in the monomer of D-alpha_14 is folded back to interact with the A and B helices, and that the form of monomeric subunit observed in the crystal structure, in which the A and B helices are continuous, does not persist in the monomer in solution. Both the dimer of D-alpha-14 and the monomer of D-alpha-14,15 appear to be twice the length of the D-alpha-14 monomer, while the frictional ration of the D-alpha-14,15 dimer is consistent with four end-to-end triple alpha-helical domains.
authors.affiliation: Department of Biochemistry, University of Sydney, NSW, Australia. G.Ralston@biochemusyd.edu.au
authors.forename: G
authors.initials: G
authors.lastname: Ralston
authors.forename: T
authors.initials: T
authors.lastname: Cronin
authors.forename: D
authors.initials: D
authors.lastname: Branton
mesh_terms: D000818:Animals; D004330:Drosophila / Q000737:chemistry; D004591:Electrophoresis, Polyacrylamide Gel; D005612:Freeze Drying; D006863:Hydrogen-Ion Concentration; D008956:Models, Chemical; D009994:Osmolar Concentration; D010446:Peptide Fragments / Q000737:chemistry* / Q000378:metabolism; D011485:Protein Binding; D011487:Protein Conformation; D012091:Repetitive Sequences, Nucleic Acid; D013049:Spectrin / Q000737:chemistry* / Q000378:metabolism; D013696:Temperature; D014461:Ultracentrifugation
chemical_list: D010446:Peptide Fragments; D013049:Spectrin