(A) Cartoon representation of the SK4 K+ ion channel shown as cylindrical helices and molecular surface depictions of CaM (pdb 6CNN). For clarity, only two of four subunits and CaMs are shown. Subunit 1 is yellow and subunit 3 is green. The N-lobe surface of CaM is vermillion, and the C-lobe surface of CaM is blue. Transmembrane helices are shown as S1–S6, and SK C-domain helices are shown as HA–HC. The peptide used in the binding studies, SKp, includes HA, HB, and part of HC. Diagonal subunits of SK4 bind a CaM in EM structure. Intracellular helix 4–5 A between S4 and S5 of transmembrane segments bind in a pocket of the N-lobe of CaM. C-domain helices HA and HB of the cross-subunit bind the C-lobe of CaM. (B) Bottom views of SK4 showing all four subunits showing CaMs bound, left side, or with CaM models removed, right side. The colors are the same as in panel A. The subunits that are not depicted in A are gray. On the right side, the SK sites that bind CaM are colored the same as the CaM lobes they contact. Panels (C–F) demonstrate the lobes of CaM can reorient to form diverse conformation states. The lobes of CaM can when bound to SK4 (pdb 6CNN) form a W-shape in C, when not bound to protein (pdb 1CLL) form an S-shape in D, when bound to an SK2 peptide (pdb 1G4Y) form a C-shape in E, or when bound to an MLCK peptide (pdb 1CDL) form an O-shape in F. (G) The N-sensor hypothesis of SK activation by CaM claims the N-lobe of CaM to be the Ca2+ sensor. The N-lobe binds a Ca2+-dependent site when Ca2+ binds to it. The C-lobe remains anchored to a site with no local change. In the NC-sensor hypothesis, both lobes of CaM are functional Ca2+ sensors. At trace Ca2+, the C-lobe dominates the interaction. At high Ca2+, a different binding mode is revealed that binds both Ca2+-bound lobes. The binding sites for each lobe of CaM need not be on the same subunit and the depiction is meant to emphasize that SK responds to Ca2+ binding to both lobes of CaM.