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Voltage-gated potassium Kv2.1 channels are widely distributed in the central nervous system, specifically in neuroendocrine and endocrine cells. Their cytoplasmic C-termini are large and carry out many important functions. Here we provide the first direct structural evidence that each C-terminal part within the Kv2.1 ion channel is formed by two distinct domains (Kv2 and CTA). We expressed and purified two C-terminal truncation mutants of a rat Kv2.1 channel, lacking the entire C-termini or the CTA domain. Single particle electron microscopy was used to obtain three-dimensional reconstructions of purified C-terminal Kv2.1 mutants at 2.0 and 2.4 nm resolution. Comparison of these structures to each other and to the low-resolution EM structure of the full-length Kv2.1 channel revealed the exact locations of cytoplasmic Kv2 and CTA domains within the tetramer. Four Kv2 domains envelop the N-terminal T1 domain. The tetramer of the CTA domains underlies the Kv2-T1 complex and may also affect the channel's surface expression. Subsequent molecular dynamics simulation and homology modeling produced open and closed structural models of the membrane part of the Kv2.1 channel.

Bacteria Tv. nitratireducens and Tv. paradoxus from soda lakes grow optimally in sodium carbonate/NaCl brines at pH range from 9.5 to 10 and salinity from 0.5 to 1.5 M Na+. Octaheme nitrite reductases (ONRs) from haloalkaliphilic bacteria of genus Thioalkalivibrio are stable and active in a wide range of pH (up to 11) and salinity (up to 1 M NaCl). To establish adaptation mechanisms of ONRs from haloalkaliphilic bacteria a comparative analysis of amino acid sequences and structures of ONRs from haloalkaliphilic bacteria and their homologues from non-halophilic neutrophilic bacteria was performed. The following adaptation strategies were observed: (1) strategies specific for halophilic and alkaliphilic proteins (an increase in the number of aspartate and glutamate residues and a decrease in the number of lysine residues on the protein surface), (2) strategies specific for halophilic proteins (an increase in the arginine content and a decrease in the number of hydrophobic residues on the solvent-accessible protein surface), (3) strategies specific for alkaliphilic proteins (an increase in the area of intersubunit hydrophobic contacts). Unique adaptation mechanism inherent in the ONRs from bacteria of genus Thioalkalivibrio was revealed (an increase in the core in the number of tryptophan and phenylalanine residues, and an increase in the number of small side chain residues, such as alanine and valine, in the core).

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