CALCIUM-DEPENDENT MOLECULES 319
disordered and do not appear in the fi nal structural renderings (see Figure
6.27 ). Peptide N - terminal residues arg1, arg2, lys3 form salt bridges with CaM
residues glu7, glu14 (N - terminal CaM) and glu120, glu127 (C - terminal CaM)
(numbering of peptide residues according to Table 6.9 ). Major CaM hydro-
phobic pocket contacts (comprised of nine met and several phe, leu, val, and
ala residues) are made between peptide residue leu4 (1) with CaM C - terminal
residues and leu13 (10) with CaM N - terminal residues. With the CaMKII
peptide, the two major hydrophobic contacts occur at residues (1) and (10)
rather than at residues (1) and (14) as seen for skMLCK and smMLCK
binding peptides. This residue (1) and (10) behavior is also seen for the CaMKI
CaM - binding peptide listed in Table 6.9. It is important to note that the Ca 2+ -
binding sites (the EF - hand, helix – loop – helix domains) in all of these structures
remain distant from the target enzyme CaM - binding peptides, enabling free
access and egress of calcium ions from their binding sites. The reference 86b
authors again mention that the CaM central linker helix is more or less dis-
rupted in complexation with different CaM - binding peptides — residues 73 – 77
are nonhelical in PDB: 1CDL and residues 74 – 83 in PDB: 1CDM — allowing
calmodulin to accommodate itself to a variety of peptide - binding sequences.
This fl exibility is believed to be a key element of calmodulin ’ s molecular rec-
ognition ability and a main contributor in the mechanism of calcium - induced
signal transduction.
The research discussed in references 85 and 86 has been summarized by
Griesinger, Krebs, and co - workers and extended to a peptide comprised of the
N - terminal portion of the calmodulin - binding domain (C20W) of the plasma
membrane calcium pump. The experimental method for structure determina-
tion was by multidimensional, heteronuclear NMR^87 (PDB: 1CFF). More
information about the calcium pump will be found in Section 6.4.2. However,
we should say now that the plasma membrane calcium pump is one of the key
enzymes controlling Ca 2+ levels within cells. The calmodulin - binding domain
of the calcium pump has a length of approximately 30 amino acid residues,
along with the capability to form an amphiphilic (polar and non - polar seg-
mented) helix. The domain has an autoinhibitory function binding to two
receptor sites near the active center of the calcium pump. The calmodulin -
binding domain ’ s N - terminal region interacts with a site near the important
phosphorylatable aspartate residue of the pump and its C - terminal region
interacts with the cytoplasmic loop between transmembrane domains 2 and 3
of the calcium pump. A unique feature of the plasma membrane calcium pump
target enzyme is that it can be activated by the C - terminal but not the N - ter-
minal half of CaM, if these two domains are isolated after trypsinolysis. The
reference 87 researchers wanted to fi nd out which part of the CaM - binding
domain of the calcium pump is suffi cient to interact solely with the C - terminal
half of calmodulin and designed three peptides, C20W, C24W, and C28W, to
answer this question. As can be seen in Table 6.9 , the N - terminal binding
domain starting with residue 1 is identical in these three peptides, but the C -
terminal domain has been truncated at residue 16 (13) of the C20W peptide.