Charge Compensation Mechanism of a Na+-coupled, Secondary Active Glutamate Transporter

• DOI: 10.1074/jbc.m112.364059
• Publication Date: 2012-06-17T21:19:58Z
• Authors: Christof Grewer; Zhou Zhang; Juddy Mwaura; Thomas Albers; Alexander Schwartz; Armanda Gameiro
• Keywords: 0303 health sciences; Binding Sites; Molecular Sequence Data; Sodium; Static Electricity; Cell Line; 03 medical and health sciences; Excitatory Amino Acid Transporter 3; Mutagenesis, Site-Directed; Potassium; Humans; Amino Acid Sequence; Protein Binding

Forward glutamate transport by the excitatory amino acid carrier EAAC1 is coupled to inward movement of three Na+ and one proton subsequent outward K+ in a separate step. Based on indirect evidence, it was speculated that cation binding sites bear negative charge. However, little known about electrostatics process. Valences calculated using Poisson-Boltzmann equation indicate charge transferred across membrane when only bound. Consistently, transient currents were observed response voltage jumps both sides membrane. Furthermore, rapid extracellular application under single turnover conditions (K+ inside) resulted current. We propose compensation mechanism, which C-terminal domain bears an overall −1.23. Charge compensation, together with distribution over many steps cycle, as well defocusing electric field, may be combined strategies used Na+-coupled transporters avoid prohibitive activation barriers for translocation.