Biology Reference
In-Depth Information
(
Londer, Dementieva, D'Ausilio, Pokkuluri, & Schiffer, 2006
). Among five
proteins (GSU2816, GSU2314, GSU0935, GSU0582, and GSU0303) con-
taining a PAS domain in their sequences, GSU0935 and GSU0582 are
characterised spectroscopically and structurally.
Both GSU0935 and GSU0582 have similar predicted topologies: a
cytoplasmic N-terminal tail followed byatransmembranehelix,aperiplas-
mic domain (ca. 135 residues), another transmembrane helix, and cytoplas-
mic domains consisting of a HAMP domain followed by a MCP domain.
X-ray crystal structures of the periplasmic PAS domains of GSU0582 and
0935 reveal that these PAS domains form a swapped dimmer (
Fig. 7.11
;
Pokkuluri et al., 2008
). In the swapped dimers, the N-terminal two helices
from one protomer associate with the
b
sheet of the other (
Pokkuluri et al.,
2008
). The haem is covalently bound in the loop between H
b
and I
b
strands and is in proximity to the N-terminal two helices of the other
protomer (
Pokkulurietal.,2008
). The location of the haem in the PAS
domain is different from that in the b-type haem-containing PAS domains
such as FixL and
Ec
Dos, suggesting that these c-type haem-containing PAS
domains adopt the different mechanisms for intramolecular signal trans-
ductions triggered by the haem upon sensing the external signal. The
PAS domains in GSU0582 and 0935 lack the FG loop that plays an impor-
tant role for the signal transductions in FixL and
Ec
Dos, which supports the
above hypothesis.
Though the crystal structures reveal the homodimeric form of the PAS
domains for GSU0935 and GSU0582, they are monomeric in diluted solu-
tion (
Pokkuluri et al., 2008; Silva, Lucas, Salgueiro, &Gomes, 2012
). Dimer
formation is observed in concentrated solution, suggesting the existence of
equilibrium between the monomer and dimer forms in solution. If this is the
case in cells, significant folding changes and conformational arrangement
should be required to the formation/dissociation of the swapped dimer.
The stability of the PAS fold would modulate domain swapping and dimer-
ization. Intriguingly, the two PAS domains for GSU0935 and GSU0582
have distinct levels of intrinsic disordered region, which are responsible
for conformational stability and signalling properties of these proteins
(
Silva et al., 2012
).
The coordination structure of the haem in GSU0935 and GSU0582 is
similar to that in
Ec
Dos (
Fig. 7.12
). The ferric haem is coordinated by
His143 in the Cys-x-x-Cys-His motif and a water molecule in
GSU0582. The haem-bound water molecule forms a hydrogen bond to
the main-chain carbonyl oxygen atom of Met60 of the other protomer.
