 |
PDBsum entry 2tn4
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
 |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Contractile system protein
|
PDB id
|
|
|
|
2tn4
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
References listed in PDB file
|
|
|
Key reference
|
|
|
Title
|
|
Structures of four ca2+-Bound troponin c at 2.0 a resolution: further insights into the ca2+-Switch in the calmodulin superfamily.
|
|
|
Authors
|
|
A.Houdusse,
M.L.Love,
R.Dominguez,
Z.Grabarek,
C.Cohen.
|
|
|
Ref.
|
|
Structure, 1997,
5,
1695-1711.
[DOI no: ]
|
|
|
PubMed id
|
|
|
|
|
|
Abstract
|
|
|
BACKGROUND: In contrast to Ca2+4-bound calmodulin (CaM), which has evolved to
bind to many target sequences and thus regulate the function of a variety of
enzymes, troponin C (TnC) is a bistable switch which controls contraction in
striated muscles. The specific target of TnC is troponin I (TnI), the inhibitory
subunit of the troponin complex on the thin filaments of muscle. To date, only
the crystal structure of Ca2+2-bound TnC (i.e. in the 'off' state) had been
determined, which together with the structure of Ca2+4-bound CaM formed the
basis for the so-called 'HMJ' model of the conformational changes in TnC upon
Ca2+ binding. NMR spectroscopic studies of Ca2+4-bound TnC (i.e. in the 'on'
state) have recently been carried out, but the detailed conformational changes
that take place upon switching from the off to the on state have not yet been
described. RESULTS: We have determined the crystal structures of two forms of
expressed rabbit Ca2+4-bound TnC to 2.0 A resolution. The structures show that
the conformation of the N-terminal lobe (N lobe) is similar to that predicted by
the HMJ model. Our results also reveal, in detail, the residues involved in
binding of Ca2+ in the regulatory N lobe of the molecule. We show that the
central helix, which links the N and C lobes of TnC, is better stabilized in the
Ca2+2-bound than in the Ca2+4-bound state of the molecule. Comparison of the
crystal structures of the off and on states of TnC reveals the specific linkages
in the molecule that change in the transition from off to on state upon
Ca2+-binding. Small sequence differences are also shown to account for large
functional differences between CaM and TnC. CONCLUSIONS: The two lobes of TnC
are designed to respond to Ca2+-binding quite differently, although the
structures with bound Ca2+ are very similar. A small number of differences in
the sequences of these two lobes accounts for the fact that the C lobe is
stabilized only in the open (Ca2+-bound) state, whereas the N lobe can switch
between two stable states. This difference accounts for the Ca2+-dependent and
Ca2+-independent interactions of the N and C lobe. The C lobe of TnC is always
linked to TnI, whereas the N lobe can maintain its regulatory role - binding
strongly to TnI at critical levels of Ca2+ - and in contrast, forming a stable
closed conformation in the absence of Ca2+.
|
|
|
|
|
|
Figure 6.
Figure 6. Conformational switch in the EF hands of the N
lobe of TnC. Stereo diagram of the EF-hand domains I (a) and II
(b) in the Ca^2+-free (yellow) and Ca^2+-bound (blue) structures
of the N lobe of TnC. The helices of a domain are reoriented
during the transition by conformational changes occurring in two
hinge regions. The mechanism of the hinges is different, in
EF-hands I and II, however. In particular, the hinge is more
extended near helix B, rather than near helix D; this is because
in EF-hand I, it involves not only residues from the b sheet but
also residues T36-E38 (orange in the Ca^2+-free structure),
which define an additional turn (cyan) of helix B when Ca^2+ is
bound.
|
|
|
|
|
|
The above figure is
reprinted
by permission from Cell Press:
Structure
(1997,
5,
1695-1711)
copyright 1997.
|
|
|
|
|
|
|
