dictyNews Electronic Edition Volume 32, number 4 February 13, 2009 Please submit abstracts of your papers as soon as they have been accepted for publication by sending them to dicty@northwestern.edu or by using the form at http://dictybase.org/db/cgi-bin/dictyBase/abstract_submit. Back issues of dictyNews, the Dicty Reference database and other useful information is available at dictyBase - http://dictybase.org. ========= Abstracts ========= 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 [richard@rice.edu] -------------------------------------------------------------------------------- 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 [thierry.soldati@unige.ch] -------------------------------------------------------------------------------- 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 [gadi@bcm.tmc.edu] ============================================================== [End dictyNews, volume 32, number 4]