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A
symbiotic way of life: 'Simple and elegant mechanism' regulates relationships
between insects and bacteria
Date:
May 5, 2014
Source:
University of Miami
Summary:
Scientists reveal how, at the cellular level, an
animal and its symbiotic bacteria work together to make up a single organismal
system.
.............................
Symbiosis is the process that occurs when two different
organisms live together to form a mutually beneficial partnership. In many
symbiotic relationships, host animals and their microbial symbionts are
partners that make up a whole -- neither one can function without the other but
together they grow and reproduce.
A study by
University of Miami researchers reveals how, at the cellular level, an animal
and its symbiotic bacteria work together to make up a single organismal system.
The study titled "Aphid amino acid transporter regulates glutamine supply
to intracellular bacterial symbionts" is published in the journal Proceedings
of the National Academy of Science (PNAS).
The findings
show how a simple mechanism allows an insect, the pea aphid, to regulate the
manufacturing of essential nutrients supplied by its symbiotic bacteria called Buchnera
aphidicola.
"We've
identified the key regulator of this symbiosis," said Alex C. C. Wilson,
associate professor of Biology in the College of Arts and Sciences and
corresponding author of the study. "It's our first real insight into the
mechanisms working at the interface of the host and symbiont."
The pea
aphid feeds on plant sap. Its diet is deficient in essential nutrients called
amino acids. The aphid can produce some amino acids on its own, but the rest it
must get from beneficial bacteria that live inside aphid cells. In turn, the
symbiotic bacteria can't produce amino acids that the aphid can make, so the
partners exchange insect-produced amino acids for symbiont-produced amino
acids.
"That
conversion of going from a diet with an inappropriate nutritional profile, to
an appropriate profile occurs in collaboration between the bacteria and the
host," Wilson said. "The question is whether the production of
nutrients changes with supply and demand and if so, how it happens."
To help
answer this question the researchers looked at amino acids that are fundamental
to the pea aphid-Buchnera symbiotic function. One of those amino acids
is glutamine, which is made in the aphid. Glutamine is important because it's
the precursor for all amino acids produced both by the aphid and by the
symbiont. The other amino acid is arginine, which is made in Buchnera and it's
deficient in the pea aphid's diet.
Glutamine is
ferried across a membrane that surrounds the cells where the bacteria lives, by
an amino acid transporter named ApGLNT1. To study this transport mechanism the
researchers used a procedure that uses frog eggs (called oocytes) to
manufacture ApGLNT1. This specialized approach is used by Charles W. Luetje,
chairman of the department of Molecular and Cellular Pharmacology in the UM
Miller School of Medicine and co-author of the study.
"My lab
specializes in the use of a technique that allows functional studies of various
receptors, channels and transporters," said Luetje. "The oocytes are
useful for expression of a wide variety of proteins and are particularly
helpful when studying difficult proteins that don't express well in other
systems."
Based on
their work figuring out what ApGLNT1 transports and where it is localized in
the pea aphid, the researchers built a model that describes how the amino acid
factory responds to supply and demand.
The findings
show that when there is a buildup of arginine in the pea aphid, arginine binds
to ApGLNT1, and inhibits glutamine uptake. Since glutamine is a precursor for
amino acids, the bacteria's synthesis of arginine is in turn reduced. Once
arginine is assimilated by the host, glutamine transport resumes and synthesis
of arginine is restored.
"To our
surprise, the transporter is a key regulator of the factory production
line," said Daniel R. G. Price, who worked on the project when he was
assistant scientist in the Department of Biology at UM and is first author of
the study.
"When
aphid demand for essential nutrients is high, the transporter imports large
amounts of precursor and the precursor is converted into essential nutrients
that are returned to the aphid," Price says. "Conversely, when there
is low aphid essential nutrient demand, little precursor is imported and the
essential nutrient production factory is shut down."
A remarkably
basic mechanism regulates the biosynthesis of symbiont-produced arginine, in
response to the needs of the pea aphid. But the model goes further than that.
"Since
ApGLNT1 localizes to the membrane of aphid cells where the bacteria resides and
because of other features peculiar to aphid metabolism, transporter ApGLNT1 not
only regulates arginine biosynthesis, but all amino acid biosynthesis,"
Wilson said. "The system is simple and elegant."
Thus amino
acid transporters play a key role in the evolutionary success of these insects.
But an important question remains: How generalizable is this regulatory
mechanism across symbiotic systems? Wilson's lab may find the answer by looking
at other sap-feeding insects with intracellular bacteria, based on an
understanding that emerged from another study from her lab. The study titled
"Dynamic recruitment of amino acid transporters to the insect-symbiont
interface" is published in the journal of Molecular Ecology.
That study
found that the presence of amino acid transporters is significantly expanded in
some sap-feeding insects relative to non sap-feeding insects. Further, these
expansions result from large-scale gene duplications that took place
independently in different sap-eating insects. Gene duplication is a process
that occurs when part of an organism's genetic material is replicated. Groups of
similar genes that share an evolutionary ancestry are called gene families.
"Given
the extensive gene duplication of the amino acid transporter gene families that
took place multiple times independently in sap-feeding insects, it makes sense
that gene duplication might be important for recruiting amino acid transporters
to mediate amino acid exchange between these insects and their symbionts,"
said Rebecca P. Duncan, doctoral student in the Department of Biology at UM and
first author of the study.
The sap-eating
insects with expanded amino acid transporters come from a common ancestor.
However, given that the genes expanded independently in each insect,
sap-feeding insects likely evolved their relationships with their symbionts
separately, as opposed to in their common ancestor. Hence, Wilson's lab can
test if their model is broadly applicable by examining the mechanism of
symbiotic regulation in the other sap-feeding insects used in this study. The
findings of these studies show that symbiotic relationships have the power to
shape animal evolution at the genetic level.
Story
Source:
The above
story is based on materials provided by University of Miami. The original article was
written by Marie Guma-Diaz and Annette Gallagher. Note: Materials may be
edited for content and length.
Journal
Reference:
- D. R. G. Price, H. Feng, J. D. Baker, S. Bavan, C. W. Luetje, A. C. C. Wilson. Aphid amino acid transporter regulates glutamine supply to intracellular bacterial symbionts. Proceedings of the National Academy of Sciences, 2013; 111 (1): 320 DOI: 10.1073/pnas.1306068111
Cite This
Page:
University of Miami. "A
symbiotic way of life: 'Simple and elegant mechanism' regulates relationships
between insects and bacteria." ScienceDaily. ScienceDaily, 5 May 2014.
<www.sciencedaily.com/releases/2014/05/140505155339.htm>.
