CALCIUM-DEPENDENT MOLECULES 323
tial differences are found in the amount of rotation and translation needed to
effect the superimposition. Calmodulin adjusts to these differences through
unwinding or expanding its central linker region. The reference 92 authors also
fi nd that glu11 is conserved in all complexes compared and forms salt bridges
in each: (1) glu 11 – arg349 for CaM/cCaMKK; (2) glu 11 – arg455 for CaM/
rCaMKKα ; (3) glu 11 – his805 for CaM/smMLCK; and (4) glu 11 – arg297 for
CaM/CaMKIIα.
The last structure to be discussed in this section is a complex between
calmodulin and the calmodulin - binding domain of myristoylated alanine - rich
C kinase (MARCKS). MARCKS is a membrane - associated protein predicted
to have an extended rod - like shape. It contains a calmodulin - binding domain
near the middle of the molecule that interacts not only with CaM, but with
other protein effectors. The same MARCKS domain contains phosphorylation
sites for several protein kinases. The CaM - binding domain consists of a char-
acteristic amphiphilic sequence with lysine clusters at both its N - and C -
terminal ends. The X - ray structure of the 19 - residue MARCKS peptide bound
to calmodulin was published by Taniguchi and co - workers in 2003 (PDB:
1IWQ).^93 Calmodulin ’ s structure in this complex shows a compact, ellipsoidal
form similar to those seen for CaM – smMLCK, CaM – skMLCK, CaM –
CaMKII α , and CaM – CaMKK complexes except that the hydrophobic
pocket in CaM ’ s N - terminal domain is shallower than for the others. Little
electron density is apparent for central linker residues 77 – 80, indicating the
expected fl exibility in this region that allows the compact CaM structure.
The MARCKS peptide was found to have a short loop (residues 164 – 166)
and a short helical region encompassing residues 157 – 163 (7 residues) at its
C - terminal end and a long loop extending from residues 156 through 148 at
its N - terminal end. The peptide ’ s structure contrasts with the helical nature
of peptides forming CaM – smMLCK, CaM – skMLCK, and CaM – CaMKII α
complexes. The peptide ’ s loop region in the CaM – MARCKS complex is
longer than that found for the CaM – CaMKK complexes and its N - terminal
loop does not curl back onto the helix as is seen in the CaM – CaMKK
complexes. Additionally, the MARCKS peptide adopts the reverse orientation
of the CaM – smMLCK, CaM – skMLCK, and CaM – CaMKII α complexes — that
is, MARCKS ’ N - terminal end interacts with CaM ’ s C - terminal domain and
its C - terminal end interacts with CaM ’ s N - terminal domain. All regions of
the peptide are involved in contacts with CaM even though the N - terminal
domain hydrophobic pocket that usually forms in Ca 2+ - saturated CaM is
almost absent in the CaM – MARCKS complex, with leu159 of the MARCKS
peptide making contact only with the N - terminal domain surface. Leu159 is
separated by only one residue from the second anchor point, phe157; however,
this residue makes the normal deep hydrophobic pocket interaction in CaM ’ s
C - terminal domain. Many ionic contacts, notably those between lys154 – glu11
(CaM helix A), lys155 – glu123 (CaM helix G), and lys154 – glu127 (helix G),
stabilize the complex. Lys154 and lys155 reside in the peptide ’ s N - terminal
loop region.