dictyNews Electronic Edition Volume 38, number 6 February 24, 2012 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. Follow dictyBase on twitter: http://twitter.com/dictybase ========= Abstracts ========= A surface glycoprotein indispensable for gamete fusion in the social amoeba Dictyostelium discoideum Yoshinori Araki*, Hideki D. Shimizu*, Kentaro Saeki, Marina Okamoto, Lixy Yamada, Kentaro Ishida, Hitoshi Sawada, and Hideko Urushihara Eukaryotic Cell, in press Sexual reproduction is essential for the maintenance of species in a wide variety of multicellular organisms, and even unicellular organisms that normally proliferate asexually possess a sexual cycle because of its contribution to increased genetic diversity. Information concerning the molecules involved in fertilization is accumulating in many species of the metazoan, plant, and fungal lineages, and the evolutionary consideration of sexual reproduction systems is now an interesting issue. Macrocyst formation in the social amoeba Dictyostelium discoideum is a sexual process in which cells become sexually mature under dark and submerged conditions and fuse with complementary mating-type cells. In the present study, we isolated D. discoideum insertional mutants defective in sexual cell fusion and identified the relevant gene, macA, which encodes a highly glycosylated, 2041-amino-acid membrane protein (MacA). Although its overall similarity is restricted to proteins of unknown function within dictyostelids, it contains LamGL and discoidin domains that are implicated in cell adhesion. Growth and development of macA-null mutants were indistinguishable from those of the parental strain. The overexpression of macA using the V18 promoter in a macA-null mutant completely restored its sexual defects. Although the macA gene encoded exactly the same protein in a complementary mating-type strain, it was expressed at a much lower level. These results suggest that MacA is indispensable for gamete interactions in D. discoideum, probably via cell adhesion. There is a possibility that it is controlled in a mating-type-dependent manner. Submitted by: Hideko Urushihara [hideko@biol.tsukuba.ac.jp] -------------------------------------------------------------------------------------- Pseudopod growth and evolution during cell movement is controlled through SCAR/WAVE dephosphorylation. Seiji Ura, Alice Y. Pollitt, Douwe M. Veltman, Nicholas A. Morrice, Laura M. Machesky, & Robert H. Insall Current Biology, in press Background SCAR/WAVE is a principal regulator of pseudopod growth in crawling cells. It exists in a stable pentameric complex, which is regulated at multiple levels that are only beginning to be understood. SCAR/WAVE is phosphorylated at multiple sites, but how this affects its biological activity is unclear. Here we show that dephosphorylation of Dictyostelium SCAR controls normal pseudopod dynamics. Results We demonstrate that the C-terminal acidic domain of most Dictyostelium SCAR is basally phosphorylated at four serine residues. A small amount of singly phosphorylated SCAR is also found. SCAR phosphorylation site mutants cannot replace SCARÕs role in the pseudopod cycle, though they rescue cell size and growth. Unphosphorylatable SCAR is hyperactive Š excessive recruitment to the front gives large pseudopods that fail to bifurcate because they continually grow forwards. Conversely, phosphomimetic SCAR is weakly active, causing frequent small, disorganised pseudopods. Even in its regulatory complex, SCAR is normally held inactive by an interaction between the phosphorylated acidic and basic domains. Loss of basic residues complementary to the acidic phosphosites yields a hyperactive protein similar to unphosphorylatable SCAR. Conclusions Regulated dephosphorylation of a fraction of the cellular SCAR pool is a key step in SCAR activation during pseudopod growth. Phosphorylation increases autoinhibition of the intact complex. Dephosphorylation weakens this interaction and facilitates SCAR activation, but also destabilizes the protein. We show that SCAR is specifically dephosphorylated in pseudopods, increasing activation by Rac and lipids and supporting positive feedback of pseudopod growth. Submitted by Robert Insall [r.insall@beatson.gla.ac.uk] ============================================================== [End dictyNews, volume 38, number 6]