dictyNews
Electronic Edition
Volume 32, number 4
February 13, 2009
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Abstracts
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Dictyostelium cells bind a secreted autocrine factor that represses
cell proliferation
Jonathan M. Choe*, Deenadayalan Bakthavatsalam*, Jonathan E. Phillips,
and Richard H. Gomer
*These authors contributed equally to this work
BMC Biochemistry, in press
Dictyostelium cells secrete the proteins AprA and CfaD. Cells lacking
either AprA or CfaD proliferate faster than wild type, while AprA or
CfaD
overexpressor cells proliferate slowly, indicating that AprA and CfaD
are
autocrine factors that repress proliferation. CfaD interacts with
AprA and
requires the presence of AprA to slow proliferation. To determine if
CfaD
is necessary for the ability of AprA to slow proliferation, whether AprA
binds to cells, and if so whether the binding requires the presence of
CfaD,
we examined the binding and effect on proliferation of recombinant AprA.
We find that the extracellular accumulation of AprA increases with cell
density and reaches a concentration of 0.3 µg/ml near a stationary cell
density. When added to wild-type or aprA¯ cells, recombinant AprA
(rAprA)
significantly slows proliferation at 0.1 µg/ ml and higher
concentrations.
From 4 to 64 µg/ ml, the effect of rAprA is at a plateau, slowing but
not
stopping proliferation. The proliferation-inhibiting activity of
rAprA is
roughly the same as that of native AprA in conditioned growth medium.
Proliferating aprA¯ cells show saturable binding of rAprA to 92,000 ±
11,000
cell-surface receptors with a KD of 0.03 ± 0.02 µg/ml. There appears
to be
one class of binding site, and no apparent cooperativity. Native AprA
inhibits the binding of rAprA to aprA¯ cells with a Ki of 0.03 µg/ml,
suggesting that the binding kinetics of rAprA are similar to those of
native AprA. The proliferation of cells lacking CrlA, a cAMP receptor-
like
protein, or cells lacking CfaD are not affected by rAprA. Surprisingly,
both cell types still bind rAprA. Together, the data suggest that AprA
functions as an autocrine proliferation-inhibiting factor by binding to
cell surface receptors. Although AprA requires CfaD for activity, it
does
not require CfaD to bind to cells, suggesting the possibility that cells
have an AprA receptor and a CfaD receptor, and activation of both
receptors
is required to slow proliferation. We previously found that crlA¯
cells are
sensitive to CfaD. Combined with the results presented here, this
suggests
that CrlA is not the AprA or CfaD receptor, and may be the receptor
for an
unknown third factor that is required for AprA and CfaD activity.
Submitted by: Richard Gomer [[log in to unmask]]
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Infection by tubercular mycobacteria is spread by nonlytic ejection from
their amoeba hosts
Monica Hagedorn1, Kyle H. Rohde2, David G. Russell2 and Thierry
Soldati1*
1 Département de Biochimie, Faculté des Sciences, Université de Genève,
Sciences II, 30 quai Ernest Ansermet, CH-1211-Genève-4, Switzerland.
2 Microbiology and Immunology, College of Veterinary Medicine, Cornell
University, Ithaca, NY 14853, USA
Science, in press
To generate efficient vaccines and cures for Mycobacterium tuberculosis,
we need a far better understanding of modes of infection, persistence
and
spreading. Host cell entry and establishment of a replication niche
are well
understood, but little is known about how tubercular mycobacteria exit
host
cells and disseminate the infection. Using the social amoeba
Dictyostelium
as a genetically tractable host for pathogenic mycobacteria, we
discovered
that M. tuberculosis and M. marinum but not M. avium are ejected from
the
cell through an actin-based structure, the ejectosome. This conserved
nonlytic spreading mechanism requires a cytoskeleton regulator from the
host and an intact mycobacterial ESX-1 secretion system. This insight
offers new directions for research into the spreading of tubercular
mycobacteria infections in mammalian cells.
Submitted by: Thierry Soldati [[log in to unmask]]
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Polymorphic members of the lag-gene family mediate kin-discrimination
in Dictyostelium
Rocio Benabentos*, Shigenori Hirose*, Richard Sucgang, Tomaz Curk,
Mariko Katoh, Elizabeth Ostrowski, Joan E. Strassmann, David C. Queller,
Blaz Zupan, Gad Shaulsky** and Adam Kuspa**
* Equal contribution
** Corresponding authors
Current Biology, in press
Self- and kin-discrimination are observed in most kingdoms of life and
are
mediated by highly polymorphic plasma membrane proteins. Sequence
polymorphism, which is essential for effective recognition, is
maintained by
balancing selection. Dictyostelium discoideum are social amoebae that
propagate as unicellular organisms, but aggregate upon starvation and
from
fruiting bodies with viable spores and dead stalk cells. Aggregative
development
exposes Dictyostelium to the perils of chimerism, including cheating,
which raises
questions about how the victims survive in nature and how social
cooperation
persists. Dictyostelids can minimize the cost of chimerism by
preferential
cooperation with kin, but the mechanisms of kin-discrimination are
largely
unknown. Dictyostelium lag-genes encode transmembrane proteins with
multiple
immunoglobulin (Ig) repeats that participate in cell-adhesion and
signaling.
Here we describe their role in kin-discrimination. We show that lagB1
and
lagC1 are highly polymorphic in natural populations and that their
sequence
dissimilarity correlates well with wild-strain segregation. Deleting
lagB1 and
lagC1 results in strain segregation in chimerae with wild-type cells,
whereas
elimination of the nearly invariant homolog lagD1 has no such
consequences.
These findings reveal an early evolutionary origin of kin-
discrimination and
provide insight into the mechanism of social recognition and immunity.
Submitted by: Gad Shaulsky [[log in to unmask]]
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[End dictyNews, volume 32, number 4]
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