Biology Reference
In-Depth Information
MCPs are usually membrane-bound proteins in which the periplasmic
and cytoplasmic domains are responsible for sensing the external signal
and signalling to CheA, respectively. The binding of a repellant or attractant
to the periplasmic sensor domain of MCPs is a cue of a conformational
change of the cytoplasmic signalling domain. As CheA forms a complex
with MCP and CheW, the conformational change of the cytoplasmic sig-
nalling domain of MCP causes a conformational change of CheA, which
results in the regulation of the autokinase activity of CheA. As O
2
can be
freely transmitted across cell membranes, several MCPs for aerotaxis are sol-
uble proteins located in cytoplasm.
6.2. MCP containing a PAS domain with b-type haem
Two types of signal transducer, HemAT and Aer, are known to act as the sen-
sor protein in microbial aerotaxis control system. HemAT is a soluble signal
transducer protein consisting of globin and MCP domains as the sensor and
regulatory domains, respectively, which senses molecular O
2
by the haem in
the globin domain (
Aono, 2011; Hou et al., 2000
). On the other hand, Aer is a
membrane-bound signal transducer protein, which adopts a flavin-containing
PAS domain as the sensor domain to sense O
2
indirectly through the redox
change of the flavin (
Taylor, Rebbapragada, & Johnson, 2001
).
A homologue of Aer, Aer2, is recently found in
Pseudomonas aeruginosa
,
which is a soluble protein consisting of a haem-containing PAS domain.
Aer2 homologues are present in some other bacteria (
Baraquet,
Th´raulaz, Iobbi-Nivol, M´jean, & Jourlin-Castelli, 2009; Osterberg,
Sk¨rfstad, & Shingler, 2010; Sarand et al., 2008
). Physiological function
of Aer2 is not clear at present. Though Hong et al. propose that Aer2 from
P. aeruginosa
is a signal transducer responsible for aerotaxis (
Hong, Kuroda,
Takiguchi, Ohtake, & Kato, 2005; Hong et al., 2004
),
Watts, Taylor, and
Johnson (2011)
argue against it.
E. coli
cells expressing Aer2 tumble con-
stantly in the presence of air and swim smoothly when O
2
is replaced by
N
2
(
Watts et al., 2011
). This response is opposite to the classic Aer-mediated
response in which
E. coli
cells tumble in response to a decrease in O
2
con-
centration. The Aer2-mediated tumbling O
2
is dependent on the extent of
receptor methylation, but the direction of the response is the same whether
or not Aer2 is methylated (
Watts et al., 2011
).
E. coli
cells expressing Aer2
also tumble in response to CO or NO, indicating that Aer2 is able to interact
with the chemotaxis system of
E. coli
to mediate repellent responses to O
2
,
CO, and NO (
Watts et al., 2011
).
