title: Electrostatic Repulsion during Ferritin Assembly and Its Screening by Ions.
abstract: Escherichia coli non-heme-binding ferritin A (EcFtnA) is a spherical cagelike protein that is composed of 24 identical subunits. EcFtnA dissociates into 2-mers under acidic conditions and can reassemble into the native structure when the pH is increased. To understand how electrostatic interactions influence the assembly reaction, the dependence of the process on ionic strength and pH was investigated. The assembly reaction was initiated by stopped-flow mixing of the acid-dissociated EcFtnA solution and high-pH buffer solutions and monitored by time-resolved small-angle X-ray scattering. The rate of assembly increased with increasing ionic strength and decreased with increasing pH from 6 to 8. These dependences were thought to originate from repulsion between assembly units (2-mer in the case of EcFtnA) with the same net charge sign; therefore, to test this assumption, mutants with different net charges (net-charge mutants) were prepared. In buffers with a low ionic strength, the rate of assembly increased with a decreasing net charge. Thus, repulsion between the assembly unit net charges was demonstrated to be an important factor determining the rate of assembly. However, the difference in the assembly rate among net-charge mutants was not significant in buffers with an ionic strength of >0.1. Notably, under such high-ionic strength conditions, the assembly rate increased with an increasing ionic strength, suggesting that local electrostatic interactions are also responsible for the ionic strength dependence of the rate of assembly and are repulsive on average.
authors.affiliation: Department of Bioinformatics, Soka University , 1-236 Tangi-machi, Hachioji, Tokyo 192-8577, Japan.
authors.forename: Daisuke
authors.initials: D
authors.lastname: Sato
authors.affiliation: Department of Bioinformatics, Soka University , 1-236 Tangi-machi, Hachioji, Tokyo 192-8577, Japan.
authors.forename: Satsuki
authors.initials: S
authors.lastname: Takebe
authors.affiliation: Department of Bioinformatics, Soka University , 1-236 Tangi-machi, Hachioji, Tokyo 192-8577, Japan.
authors.forename: Atsushi
authors.initials: A
authors.lastname: Kurobe
authors.affiliation: Department of Bioinformatics, Soka University , 1-236 Tangi-machi, Hachioji, Tokyo 192-8577, Japan.
authors.forename: Hideaki
authors.initials: H
authors.lastname: Ohtomo
authors.affiliation: Department of Bioinformatics, Soka University , 1-236 Tangi-machi, Hachioji, Tokyo 192-8577, Japan.
authors.forename: Kazuo
authors.initials: K
authors.lastname: Fujiwara
authors.affiliation: Department of Bioinformatics, Soka University , 1-236 Tangi-machi, Hachioji, Tokyo 192-8577, Japan.
authors.forename: Masamichi
authors.initials: M
authors.lastname: Ikeguchi
mesh_terms: D029968:Escherichia coli Proteins / Q000737:chemistry* / Q000235:genetics; D005293:Ferritins / Q000737:chemistry* / Q000235:genetics; D006863:Hydrogen-Ion Concentration; D007477:Ions; D008958:Models, Molecular; D009154:Mutation; D009994:Osmolar Concentration; D011487:Protein Conformation; D055503:Protein Multimerization; D021122:Protein Subunits / Q000737:chemistry; D053838:Scattering, Small Angle; D055672:Static Electricity; D014965:X-Rays
chemical_list: D029968:Escherichia coli Proteins; D007477:Ions; D021122:Protein Subunits; C534454:ferritin A, E coli; D005293:Ferritins