Secondary structure, stability and tetramerisation of recombinant K(V)1.1 potassium channel cytoplasmic N-terminal fragment

Identification of a novel tetramerization domain in large conductance K(ca) channels

More than 50 genes are known to encode K(+) channel monomers and can coassemble to form hetero-tetrameric K(+) channels. However, only a subset of possible monomer combinations come together to form functional ion channels. The assembly and tetramerization of appropriate channel monomers is mediated by association domains (ADs). To identify such domains in human large-conductance Ca(2+)-activated K(+) channels (hSlo1), we screened hSlo1 domains for self-association using yeast two-hybrid assays. Putative ADs were subjected to functional assays in Xenopus oocytes and further characterized by coprecipitation, native gel electrophoresis, and sucrose density gradient centrifugation assays. This led to the identification of a single intracellular association domain localized near the channel pore and required for channel function. We conclude that this novel tetramerization domain, referred to as BK-T1, promotes the assembly of hSlo1 monomers into functional K(Ca) channels.

A carboxy-terminal alpha-helical segment in the rat skeletal muscle voltage-dependent Na+ channel is responsible for its interaction with the amino-terminus

Cytoplasmic segments of the adult rat skeletal muscle sodium channel alpha-subunit (rSkM1) comprise a major portion (approximately 40%) of the total protein and are involved in channel functions both general, such as inactivation, and isoform-specific, for example, protein kinase A modulation. Far ultraviolet circular dichroism measurements of synthetic peptides and overexpressed fusion proteins containing individual channel cytoplasmic segments suggest that cytoplasmic domains of rSkM1 contain ordered secondary structures even in the absence of adjoining transmembrane segments. Intrinsic fluorescence experiments with a nested set of carboxy-terminal deletion proteins confirm a specific interaction between the channel's amino- and carboxy-termini and identify residues 1716-1737 in the carboxy-terminus as the region that binds to the amino-terminus. Circular dichroism measurements suggest that this same region is organized as an alpha-helix and that electrostatic forces may contribute to this association. The interaction of the amino- and carboxy-termini is not accompanied by secondary structure changes detectable by circular dichroism spectroscopy, but a decrease in intrinsic fluorescence indicates that this association is accompanied by a change in the environment of Trp1617.