CALCIUM-DEPENDENT MOLECULES 325
complexes the specifi c binding interface between the two proteins is hydro-
phobic. In the CaM N - lobe, residues phe12, phe19, val35, met36, leu39, phe68,
met 71, and met72 create a hydrophobic patch that interacts with CaMBD
residues leu480, leu483, and ala484. In this case, the leu480 residue is the
anchor residue reaching far into the N - terminal hydrophobic pocket. Interac-
tions of CaMBD units with the calmodulin ’ s C - terminal domains in the CaM –
CaMBD complex are different from those seen previously because of the
distinct conformation of the calcium - free C - lobe. The calcium - free C - lobe
almost adopts the “ closed ” conformation seen for apo - calmodulin described
in Section 6.3.2.2 except that the angle between the G and H helices opens up
to create several smaller and discrete hydrophobic binding pockets for CaMBD
binding. The CaMBD side - chain residues ala425, leu428, and trp432 bind to
calmodulin residues glu127, met124, phe141, and met144. Additionally, leu428
plugs a hydrophobic patch in the CaM C - lobe formed by phe89, phe92, leu105,
and met109. Schumacher et al.^94 extend the structural analysis of the CaM/
CaMBD system to propose a model for K + - channel opening facilitated by the
calmodulin/SK - channel calmodulin - binding domain interactions. First, it is
necessary to know that the Ca 2+ - activated K + channels (SK channels) are
voltage - independent and are gated solely by intracellular Ca 2+. The reference
94 authors propose a chemomechanical model in which calcium ion addition
and subsequent CaM – CaMBD complexation changes the orientation of SK -
channel S6 helices attached to the CaMBD units, opening them for ion passage.
Figure 6 of reference 94 illustrates the chemomechanical model.
Finally in this section, Vetter and Leclerc, reference 88 , discuss crystal struc-
tures of the C - terminal fragment (residues 291 – 800) of the Bacillus anthracis
exotoxin. The anthrax exotoxin (so - called edema factor, EF) is a calmodulin
(CaM) - dependent adenylate cyclase. It enters host cells through a transporter
produced by the anthrax pathogen. EF aquires calmodulin inside the cell and
then converts large amounts of ATP to cyclic AMP (cAMP). The X - ray crystal-
lographic structures of the anthrax exotoxin (edema factor, EF) alone (PDB:
1K8T), EF in complex with calmodulin (PDB: 1K93), and EF in complex with
calmodulin and an ATP analog (PDB: 1K90) have been determined by Chester
L. Drum and associates.^91 The structures are important because one can
compare the exotoxin structure as it changes through complexation with
calmodulin (PDB: 1K93) and then with CaM plus 3 ′ dATP, an inactive ATP
analog (PDB: 1K90). These are also the fi rst structures of CaM complexed
with a catalytically functional CaM – target enzyme. Drum and co - workers
found that the structure of EF alone differs substantially from that of CaM - EF,
but very few further changes are seen when adding the ATP analog. Therefore,
it is assumed that the changes in CaM – EF structure are almost all due to the
binding of CaM. CaM, in an extended conformation similar to Ca 2+ - CaM
(PDB: 1CLL) and with its central helix unwound between residues 76 – 78, is
inserted deeply between two domains of the exotoxin. The CaM molecule
contains two Ca 2+ ions in its C - terminal domain and no Ca 2+ ions in its N -
terminal domain. The N - terminal domain is in the “ closed ” conformation and