The interplay between ion channel a- and b-subunits combined with post-translational modifications determines the final electrical properties of ionic currents in native tissue. The Kvb family was identified for their interaction with members of the Kv1 and the Kv4 family while the KChIP family was initially identified as partners of the Kv4 channels. However, recent evidence indicates an interaction between Kv1.5 and KChIP subunits. Structural data suggest that simultaneous binding of both KChIP and Kvb subunits to Kv1.5 or Kv4.2 is possible with both subunits attaching to a different part of the channel. Based on co-IP experiments of tagged subunits a direct interaction between KChIP and Kvb subunits was shown. This novel interaction was confirmed with positive in vivo FRET between Kvb2.1 subunits tagged with YFP and KChIP subunits tagged with CFP. To test for the presence of a trimeric assembly of Kv1.5 (or Kv4.2) channels, KChIP subunits and Kvb subunits, FRET experiments were designed with different combinations of the tags. Finally, electrophysiological experiments were performed upon transfection of KChIPs in Ltk- cells expressing Kv1.5 and Kvb2.1 subunits. The presence of transfected KChIP subunits did not affect the electrophysiological properties of Kv1.5 co-expressed with Kvb. The results indicate complex interactions between different classes of auxiliary channel subunits.