The BTB domains of the potassium channel tetramerization domain proteins prevalently assume pentameric states

Potassium channel tetramerization domain-containing (KCTD) proteins are involved in fundamental physio-pathological processes. Here, we report an analysis of the oligomeric state of the Bric-a-brack, Tram-track, Broad complex (BTB) domains of seven distinct KCTDs belonging to five major clades of the family evolution tree. Despite their functional and sequence variability, present electron microscopy data highlight the occurrence of well-defined pentameric states for all domains. Our data also show that these states coexist with alternative forms which include open pentamers. Thermal denaturation analyses conducted using KCTD1 as a model suggest that, in these proteins, different domains cooperate to their overall stability. Finally, negative-stain electron micrographs of KCTD6 BTB in complex with Cullin3 show the presence of assemblies with a five-pointed pinwheel shape.