Effect of selectivity filter radius on Ca 2+ vs. Na + selectivity

1. THE ROLE OF DIELECTRICS IN THE SELECTIVITY OF CALCIUM CHANNELS Dezső Boda1,2, Mónika Valiskó1,Dirk Gillespie2, Wolfgang Nonner3, Douglas Henderson4, and Bob Eisenberg2 1University of Veszprém, Veszprém, Hungary, 2Rush University Medical Center, Chicago, 3University of Miami School of Medicine, 4Brigham Young University, Provo, Utah Ca2+ Ca2+ Ca2+ Ca2+ Na+ Na+ Na+ Na+ O1/2- O1/2- O1/2- O1/2- The negative charges of the protein charges induce negative charge on the protein/lumen interface, attracting more cations into the channel. Abstract Effect of protein dielectric constant on Ca2+ vs. Na+ selectivity Effect of selectivity filter radius on Ca2+ vs. Na+ selectivity Calcium channels range in their affinity for Ca2+. For example, the L-type calcium channel has micromolar affinity while the ryanodine receptor has millimolar affinity. On the other hand, both of these channels have the chemically-similar EEEE and DDDD amino acid motifs in their selectivity filters. To understand the physical mechanism for this range of affinities given similar amino acids in the selectivity filter, we use Monte Carlo simulations to assess the binding of monovalent and divalent ions in the selectivity filter of a model calcium channel. We find that the dielectric properties of the protein surrounding the permeation pathway greatly influence the Ca2+ affinity, with a low-dielectric protein greatly increasing Ca2+ affinity. This counterintuitive result occurs because calcium channel selectivity filters are lined with negatively-charged amino acids (EEEE or DDDD). With a low-dielectric protein, these negative charges induce more negative charges at the protein/lumen interface. This effectively increases the negative charge of the protein and increases the affinity of the channel for Ca2+. If no negative protein charges were present, cations would induce positive charges at the protein/ lumen interface, creating an energy barrier large enough to prevent ion flux through the channel. Decreasing the pore radius significantly increases the dielectric effect and selectivity. Low protein dielectric constant ε attracts cations into the channel Decreasing protein dielectric constant ε attracts Ca2+ into the channel disproportionately over Na+ Decreasing selectivity filter radius increases charge density Decreasing selectivity filter radius decreases Na+ occupancy. At ε = 10, decreasing the radius increases Ca2+ occupancy. Optimal Ca2+ vs. Na+ selectivity Decreasing both radius and protein dielectric constant significantly increases Ca2+ vs. Na+ selectivity cross-point vs. filter radius R = 4.5 A cross-point vs. protein dielectric constant Ca2+ Monte Carlo simulations Ca2+ Ca2+ Ca2+ Na+ Na+ Na+ Na+ We study equilibrium binding selectivity with Monte Carlo simulations with ions modeled as charged, hard spheres: Ca2+ Ca2+ • the bath contains 100 mM NaCl • CaCl2 is added to the bath • the ions are in dielectric 80 • the membrane has dielectric constant 80 (making it 2 does not change results and increases the computation time) • the dielectric constant of the protein is varied • The channel geometry Conclusion Protein dielectric constant and pore radius are two parameters that can be changed by evolution to regulate Ca2+ selectivity. We showed that decreasing the radius and the protein dielectric constant individually increases selectivity. Decreasing them simultaneously, the effect is significantly larger; selectivity is shifted from millimolar [Ca2+] into the micromolar range. Na+ Na+ Our measure of selectivity (channel affinity) is the “cross-point”: the [Ca2+] where the number of Ca2+ and Na+ in the selectivity filter are equal. The cross-point has a minimum (i.e., the selectivity has a maximum) because decreasing the protein dielectric constant preferentially attracts Ca2+ over Na+ until Ca2+ occupancy saturates and Na+ occupancy continues to increase. Decreasing radius increases induced charge and increases packing in the selectivity filter R=4.5A, ε=80 R=3.5A, ε=80 R=4.5A, ε=10 R=3.5A, ε=10 Decreasing protein dielectric coefficient increases the negative charge of selectivity filter References Charge/space competition in Ca2+ channels W. Nonner, L. Catacuzzeno, and B. Eisenberg, Biophys. J.79, 1976 (2000). Methods D. Boda, D. Gillespie, W. Nonner, et al., Phys. Rev. E69, 046702 (2004).