CALCIUM-DEPENDENT MOLECULES 321
the crucial ATP - binding domain of both CaMKI and the Ca 2+ pump. Binding
of calmodulin through hydrophobic interaction with trp4 (1) is proposed as
the fi rst event in a cascade that removes the autoinhibitory CaM - binding
domain from its position on the ATP binding domain of the target enzyme,
thus activating the enzyme for its physiological purpose.
More recently, several different calmodulin binding modes to target enzymes
have been described. Vetter and Leclerc discuss several of these in their 2003
review article.^88 These authors use the reference 87 numbering system for the
target enzyme binding peptides, as seen in Table 6.9. We have discussed the
collapsed (compact) structures exhibited by calmodulin when complexed
with target peptides skMLCK (reference 85 , PDB: 2BBM), smMLCK (refer-
ence 86a , PDB: 1CDL), and CaMKII α (reference 86b , PDB: 1CDM) and
the extended calmodulin structure when complexed with the C20W peptide
(reference 87 , PDB: 1CFF). In all of these complexes, the target enzyme
binding peptide forms anα - helix upon calmodulin binding. Vetter and Leclerc
review these structures and go on to discuss calmodulin complexes with
Ca2+ /CaM - dependent kinase kinase,^89 the Ca 2+ activated K + channel,^90
and the anthrax exotoxin (edema factor), a calmodulin - dependent adenylate
cyclase.^91
Ca 2+ /CaM - dependent kinase kinase (CaMKK) is a Ca 2+ /CaM - dependent
serine/threonine kinase that phophorylates other CaM - dependent kinases. (A
kinase is a type of enzyme that transfers phosphate groups from high - energy
donor molecules, such as ATP, to specifi c target molecules — substrates — that
are often amino acid residues of the target protein. CaMKK is at the top of a
cascade of phosphorylation events. Upon its activation by calmodulin, CaMKK
subsequently phosphorylates two serine/threonine kinases, CaM - kinase I
(CaMKI) and CaM - kinase IV (CaMKIV). CaMKI and CaMKIV are them-
selves calmodulin - dependent enzymes. The calmodulin - binding domain of
CaMKK is different from those previously discussed in that the CaM binding
residues are further apart than those for skMLCK, smMLCK, and CaMKII α.
Also a cluster of basic residues (principally arginines (R) and lysines (K)) are
located near the C - terminal anchor residue of CaMKK (phe16 in Table 6.9 )
rather than near the N - terminal anchor residue (trp1 or leu1 in Table 6.9 ) of
the previously discussed CaM - binding peptides.
Many more structure determinations of CaM - binding peptides have been
carried out. For instance, the NMR structure determined by Ikura and
co - workers for a CaM/CaMKK (from rat) complex (PDB: 1CKK) shows a
calmodulin collapsed structure similar to those of CaM/smMLCK, CaM/
skMLCK, and CaM/CaMKII α with the rCaMKK peptide cradled between
calmodulin ’ s C - and N - terminal domains.^89 However, two features are differ-
ent for CaM/rCaMKK. The fi rst is that the peptide is bound in an inverted
position compared to the others — that is, the N - terminal end of the rCaMKK
peptide binds to CaM ’ s N - terminal end and the C - terminal end binds to the
C - terminal end rather than vice versa. This factor appears related to the clus-
ters of basic residues on the target enzyme binding peptide — that is, when the