Membrane Anchoring and Interaction between Transmembrane Domains are Crucial for K+ Channel Function

Abstract

The small viral channel Kcv is a Kir-like K ؉ channel of only 94 amino acids. With this simple structure, the tetramer of Kcv represents the pore module of all complex K ؉ channels. To examine the structural contribution of the transmembrane domains (TMDs) to channel function, we performed Ala scanning mutagenesis of the two domains and tested the functionality of the mutants in a yeast complementation assay. The data reveal, in combination with computational models, that the upper halves of both TMDs, which face toward the external medium, are rather rigid, whereas the inner parts are more flexible. The rigidity of the outer TMD is conferred by a number of essential aromatic amino acids that face the membrane and probably anchor this domain in the bilayer. The inner TMD is intimately connected with the rigid part of the outer TMD via ⅐⅐⅐ interactions between a pair of aromatic amino acids. This structural principle is conserved within the viral K ؉ channels and also present in Kir2.2, implying a general importance of this architecture for K ؉ channel function.K ϩ channels are transmembrane proteins, which catalyze the selective and regulated flux of potassium ions across membranes. A breakthrough in understanding of structure/function correlates in these important proteins occurred with the highresolution structures determined for several K ϩ channels (1-4). Many functional properties such as selectivity and gating of K ϩ channels are now understood on the basis of the specific architecture of these channel proteins. Still, many aspects of function and regulation cannot be explained only on the basis of the protein structure. The performance of the protein also depends on the lipid environment and on the organization of the protein in this environment. Indeed, there is increasing evidence that many different properties of K ϩ channels are depending on the interaction between the protein and the surrounding host membranes (5-7). As a consequence of this dependence, many amphiphilic drugs, which target ion channels, have dual effects, one effect being directly related to an interaction with the protein and a secondary effect being mediated by modification of the bilayer properties (8).A good model system for understanding the interplay of membrane proteins with the membrane is provided by the viral potassium channel Kcv (9, 10). This Kir-like potassium channel with two transmembrane domains is, with only 94 amino acids, very small. The protein is almost completely embedded in the membrane. Only a small, 15 amino acid-long domain at the N terminus and a short turret domain between the two transmembrane domains stick out of the membrane into aqueous solution (11). It can be assumed that in this arrangement any protein/membrane interaction strongly reflects back on function.Unbiased information on the structural significance of the two transmembrane domains (TMDs) 2 and their importance for channel function can be obtained by an alanine scan of the relevant domains. In this approach, each amino acid in a primary seq...

Viruses infecting marine picoplancton encode functional potassium ion channels

“…The present experimental verification of their function as K þ selective channels makes them the smallest proteins known to form a functional K þ channel. Sequences alignments and analysis of specific amino acids, which were identified in previous studies as functionally important (Gebhardt et al, 2011, support a clear phylogenetic separation of channels from fresh water viruses versus salt/sea water viruses. The phylogenetic relationships, however, provide no evidence for a co-evolution between viruses and their hosts; this finding agrees with previous studies on the relationship between host and virus K þ channels (Hamacher et al, 2012.…”

“…The cationic amino acid d is a "snorkeling" amino acid and required for placing the TMD in the bilayer . Finally a pair of Phe or Met (e) form together with a His (f) in the second TMD, a π: π-stacking or C-H: π interaction; this interaction attaches the inner TMD to the outer TMD (Gebhardt et al, 2011). The listed amino acids are conserved among more than 80 Kcv type channels from viruses, which infect fresh water algae (Thiel and Van Etten, unpublished data).…”

“…Previous studies on structure function relations established that some amino acids are very important for Kcv channel activity (Gebhardt et al, 2011; these significant amino acids are labeled with small letters (a-f) in Fig. 5.…”

“…5. The aromatic amino acids a-c are required to anchor the first TMD in the lipid bilayer (Gebhardt et al, 2011). The cationic amino acid d is a "snorkeling" amino acid and required for placing the TMD in the bilayer .…”

Exploring the Viral Channel KcvPBCV-1 Function via Computation

“…(2012)F30, H83Mutating these residues is not tolerated in yeast growth assays. Gebhardt et al. (2011) …”