Biology Reference
In-Depth Information
domain is driven by the coupled haem-binding PAS domain, but a mutual
communication between these two domains is also present. The spin-state
of the haem group controls the activation of the kinase, and a phosphate-
dependent feedback controls the ligand affinity of the haem (
Gilles-
Gonzalez, Gonzalez, & Perutz, 1995; Rodgers, Lukat-Rodgers, & Barron,
1996; Sousa et al., 2013
). Next to the haem-group spin-state regulation, an
alternative inactivation mechanism has been proposed in which an aberrant
disulfide bridge is formed in the homo-dimer (
Akimoto, Tanaka,
Nakamura, Shiro, & Nakamura, 2003
) and might also be involved in the
ligand discrimination.
Although the expression of
fix
genes has been reported in a number of
bacterial strains (
Anthamatten & Hennecke, 1991; Anthamatten, Scherb, &
Hennecke, 1992; Crosson, McGrath, Stephens, McAdams, & Shapiro,
2005; David et al., 1988; de Philip, Batut, & Boistard, 1990; D'Hooghe,
Michiels, & Vanderleyden, 1998; D'Hooghe et al., 1995; Girard et al.,
2000; Iniesta et al., 2010; Kaminski & Elmerich, 1991
), to date only two
of these have been extensively characterized with multiple approaches,
which are the transmembrane
S. meliloti
FixL (
Sm
FixL) (
Lois et al., 1993
)
and the soluble
Bradyrhizobium japonicum
FixL (
Bj
FixL) (
Anthamatten &
Hennecke, 1991
).
Sm
FixL is a 505-residue-long protein that contains the optional trans-
membrane domain, organized in four segments. It localizes to the cytoplas-
mic side of the inner bacterial membrane (
Lois et al., 1993
). The presence or
absence of different domains of the protein influences the ligand-binding
kinetics to the haem group. The deletion or retention of the kinase domain
causes the rate and the stability of the gas-haem complexes to diminish,
suggesting a direct or indirect influence of the second domain on the steric
interactions between the iron-bound ligand and the amino acid side chains
of the haem-pocket (
Gilles-Gonzalez et al., 1994; Miyatake et al., 1999;
Rodgers et al., 1996; Rodgers, Lukat-Rodgers, & Tang, 2000
).
A closer look at the haem-binding domain reveals the possible role of
Ile209 and Ile210 in signal transduction. These two residues are in the
neighbourhood of the ligand bound to the haem group and, when O
2
is pre-
sent, are displaced by steric repulsion. This movement is suggested to be
responsible for the conformational changes in the FG-loop, which, in turn,
influences the kinase structure, thus regulating its activation (
Miyatake et al.,
2000; Mukai, Nakamura, Nakamura, Iizuka, & Shiro, 2000
). Time-resolved
Resonance Raman spectra measured on the full length and on the truncated
forms of
Sm
FixL during both O
2
and CO dissociation, as well as mutational
