Calmodulin (CAM) is inferred to be encoded in all eukaryotes and
expressed in most cells. In its four calcium form it activates over
twenty target enzymes or structural proteins . Pairs of domains 1&2
are connected to lobe 3-4 by an eight residues linker. Helix F2, the
linker, and helix E3 form a seven turn _-helix. The linker serves as a
flexible tether. In the apo-form both lobes are in the closed
conformation and the tether is extended; CAM is then dumbbell shaped.
In the Ca4-form the tether is bent, both lobes are in an open
conformation, and the two lobes enfold a helix of the target protein
. The ten subfamilies (CAM, TNC, ELC, RLC TPNV, CLAT, SQUD, CDC,
CAL, and CAST) are congruent (figure 5).