Small K+ channels are model proteins for detecting basic interactions between membrane proteins and their surrounding bilayer with relevance for structure and function

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Structure/function correlates of membrane proteins can only be fully understood with profound knowledge on the complex interplay between proteins with their lipid environment. In the last decade much work was devoted to understand the basic architecture of transmembrane domains (TMDs) and their embedding in lipid bilayers. A systematic research with simple synthetic one membrane spanning proteins has in this context provided many of the basic rules for the amino acid composition of a-helical TMDs. The goal of the present project is to extend this systematic approach and examine structure/function correlates of membrane proteins by entering a higher level of complexity, which includes a measurable function. Ideal tools for this endeavor are provided by small viral K+ channels with different TMDs. A subunit of these functional channel proteins is built from two TMDs, which are just long enough to span a membrane. From a bulk of experimental and structural work we already know essential properties of the TMDS of these channels, which are important for an anchoring of the proteins in the membrane. A successful realization of the project is further supported by experimental progress, which allows us to produce channel proteins by in vitro translation in so called nano-discs. From there they can be reconstituted in various types of lipid bilayers and their function recorded as single channel fluctuations. With this solid background we propose to systematically examine the functional properties of model channel proteins with different TMDs in lipid bilayers with distinct physic chemical flavors like variable bilayer thickness, different head groups and with or without cholesterol. This work will be complemented by structural work in which small and wide angle X-ray scattering methods are used to determine the effect of the channel proteins on the geometry of the lipid bilayer. High-resolution microscopy will be employed to examine the potential formation of channel clusters in different membranes. Taken together these complementary experiments will provide a solid data base for an understanding of structure and function relations of simple channel proteins in variable lipid environments. We will obtain detailed information on the influence of different membrane properties on K+ channel function including gating and unitary conductance. The data will further show how the two partners, e.g. the membrane and the protein, compensate mismatches in the length of TMDs with respect to bilayer thickness without compromising function. The fact that the general structure of the viral K+ channels is similar to the pore structure of complex channels opens the possibility to extrapolate data from this research to physiologically relevant K+ channels.