SWISS-MODEL: homology modelling of protein structures and complexes.

Homology modelling has matured into an important technique in structural biology, significantly contributing to narrowing the gap between known protein sequences and experimentally determined structures. Fully automated workflows and servers simplify and streamline the homology modelling process, also allowing users without a specific computational expertise to generate reliable protein models and have easy access to modelling results, their visualization and interpretation. Here, we present an update to the SWISS-MODEL server, which pioneered the field of automated modelling 25 years ago and been continuously further developed. Recently, its functionality has been extended to the modelling of homo- and heteromeric complexes. Starting from the amino acid sequences of the interacting proteins, both the stoichiometry and the overall structure of the complex are inferred by homology modelling. Other major improvements include the implementation of a new modelling engine, ProMod3 and the introduction a new local model quality estimation method, QMEANDisCo. SWISS-MODEL is freely available at https://swissmodel.expasy.org.

Initial steps of inactivation at the K+ channel selectivity filter.

K(+) efflux through K(+) channels can be controlled by C-type inactivation, which is thought to arise from a conformational change near the channel's selectivity filter. Inactivation is modulated by ion binding near the selectivity filter; however, the molecular forces that initiate inactivation remain unclear. We probe these driving forces by electrophysiology and molecular simulation of MthK, a prototypical K(+) channel. Either Mg(2+) or Ca(2+) can reduce K(+) efflux through MthK channels. However, Ca(2+), but not Mg(2+), can enhance entry to the inactivated state. Molecular simulations illustrate that, in the MthK pore, Ca(2+) ions can partially dehydrate, enabling selective accessibility of Ca(2+) to a site at the entry to the selectivity filter. Ca(2+) binding at the site interacts with K(+) ions in the selectivity filter, facilitating a conformational change within the filter and subsequent inactivation. These results support an ionic mechanism that precedes changes in channel conformation to initiate inactivation.

Mechanism of activation at the selectivity filter of the KcsA K+ channel.

Potassium channels are opened by ligands and/or membrane potential. In voltage-gated K+ channels and the prokaryotic KcsA channel, conduction is believed to result from opening of an intracellular constriction that prevents ion entry into the pore. On the other hand, numerous ligand-gated K+ channels lack such gate, suggesting that they may be activated by a change within the selectivity filter, a narrow region at the extracellular side of the pore. Using molecular dynamics simulations and electrophysiology measurements, we show that ligand-induced conformational changes in the KcsA channel removes steric restraints at the selectivity filter, thus resulting in structural fluctuations, reduced K+ affinity, and increased ion permeation. Such activation of the selectivity filter may be a universal gating mechanism within K+ channels. The occlusion of the pore at the level of the intracellular gate appears to be secondary.