dictyNews Electronic Edition Volume 31, number 17 December 5, 2008 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 ========= Subcellular localization of ammonium transporters in  Dictyostelium discoideum Janet H. Kirsten (1), Yanhua Xiong (1), Carter T. Davis (2), Charles K. Singleton (1) (1) Department of Biological Sciences, Vanderbilt University, VU Station B 351634, Nashville TN 37235-1634, USA (2) LSU School of Medicine – New Orleans, 2020 Gravier Street, New Orleans, LA 70112, USA BMC Cell Biology, in press Background With the exception of vertebrates, most organisms have plasma membrane bound ammonium transporters which primarily serve to import a source of nitrogen for nutritional purposes. Dictyostelium discoideum has three  ammonium transporters, Amts A, B and C. Our present work used fluorescent fusion proteins to determine the cellular localization of the  Amts and tested the hypothesis that the transporters mediate removal of  ammonia generated endogenously from the elevated catabolic activity  common to many protists. Results Using RFP and YFP fusion constructs driven by the actin 15 promoter, we found that the three ammonium transporters were localized on the plasma membrane and on subcellular organelles. AmtA and AmtB were localized on the membranes of endolysosomes and phagosomes, with  AmtB further localized on the membranes of contractile vacuoles. AmtC  alsowas localized on subcellular organelles when it was stabilized by coexpression with either the AmtA or AmtB fusion transporter. The three ammonium transporters exported ammonia linearly with regard to time during the first 18 hours of the developmental program as revealed by reduced export in the null strains. The fluorescently tagged transporters rescued export when expressed in the null strains, and thus they were functional transporters. Conclusions Unlike ammonium transporters in most organisms, which import NH3/NH4+ as a nitrogen source, those of Dictyostelium export ammonia/ammonium as a waste product from extensive catabolism of exogenously derived and endogenous proteins. Localization on proteolytic organelles and on the neutral contractile vacuole suggests that Dictyostelium ammonium transporters may have unique subcellular functions and play a role in the maintenance of intracellular ammonium distribution. A lack of correlation between the null strain phenotypes and ammonia excretion properties of the ammonium transporters suggests that it is not the excretion function that is important for coupling ammonia levels to the slug versus culmination choice, but rather a sensor and/or signaling function of these proteins that is important. Submitted by: Janet Kirsten [Jan.Kirsten@vanderbilt.edu] -------------------------------------------------------------------------------- Role of an expansin-like molecule in Dictyostelium morphogenesis and  regulation of its gene expression by the STAT protein Dd-STATa Shun Ogasawara1, Nao Shimada1, 2 and Takefumi Kawata1, 3 1 Department of Biology, Faculty of Science, Toho University, 2-2-1 Miyama,  Funabashi, Chiba 274-8510, Japan 2Present Address: Department of Basic Science, Graduate School of Arts and  Sciences, University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo, 153-8902, Japan 3Corresponding author E-mail: tkawata@bio.sci.toho-u.ac.jp Devleop. Growrh Differ., in press Expansins are proteins involved in plant morphogenesis, exerting their  effects on cellulose to extend cell walls. Dictyostelium is an organism  that possesses expansin-like molecules, but their functions are not  known. In this study, we analysed the expL7 (expansin-like 7) gene,  which has been identified as a putative target of Dd-STATa, a  Dictyostelium homologue of the metazoan signal transducer and  activator of transcription (STAT) proteins. Promoter fragments of the  expL7 were fused to a lacZ reporter and the expression patterns  determined. As expected from the behaviour of the endogenous  expL7 gene, the expL7/lacZ fusion gene was downregulated in  Dd-STATa null slugs. In the parental strain, the expL7 promoter was  activated in the anterior tip region. Mutational analysis of the promoter  identified a sequence that was necessary for expression in tip  cells. In addition, an activator sequence for pstAB cells was  identified. These sequences act in combination with the repressor  region to prevent ectopic expL7 expression in the prespore and  prestalk regions of the slug and culminant. Although the expL7 null  mutant showed no phenotypic change, the expL7 overexpressor  showed aberrant stalk formation. These results indicate that the  expansin-like molecule is important for morphogenesis in Dictyostelium. Submitted by: Takefumi Kawata [tkawata@bio.sci.toho-u.ac.jp] ============================================================== [End dictyNews, volume 31, number 17]