Web sequence gazing

thinking to clarify complex problems. My introduction to Tilney’s Listeria papers came when I sought a post-doctoral position with Daniel Louvard at the Institut Pasteur in Paris. I had previously studied mitosis and wanted to learn about the behaviour of the actin cytoskeleton during mitosis, in particular at cytokinesis. I was directed to Louvard’s associate, Evelyne Friederich, who had recently set up a project based upon studies of Listeria. She and others transfected human cultured cells with actA cDNAs with dramatic effects: the cells filled up with actin filaments and sent off numerous membrane-bound cytoplasmic extensions [2]. This result suggested that human cells have proteins that react specifically with ActA, so they might also have a natural ligand — a human ActA-like protein. The research project on offer was to identify ActA-like proteins of human cells using ActA-specific antibodies — a long way from my aim of studying cytokinesis. But the project was enticing for several reasons. The laboratory had considerable experience in moving from cross-reacting antibodies to models of how a protein might function, and the evolutionary origins of the actA gene were in question because only two of the six Listeria species carry actA — the same two species that can form comets — suggesting that the gene might at one time have been acquired from another genome, possible mammalian. But it was a literature search on the subject that helped close the deal. Many of the papers on ActA cited Tilney in their introduction, including the seminal 1989 paper [1]. Upon reading this paper and remembering Tilney’s seminars at MBL, I dropped the idea of studying cytokinesis to join Friederich’s group and study ActA instead. In the 1989 paper, Tilney and Portnoy wrote that their findings “should be exciting to cell biologists who want to know how actin filaments become organised in cells” [1]. Many authors throw in such a phrase about their results, but this time it was true. Studies of ActA have already uncovered a number of important cytoskeletal proteins: with Mary Beckerle, we found zyxin, while Trinad Chakraborty, Ulrich Walter and Jürgen Wehland and colleagues found that VASP (vasodilator stimulated phosphoprotein) binds to zyxin and to ActA, and the somewhat mysterious Arp2/3 complex of proteins resurfaced in Tim Mitchison’s laboratory as being required for ActA-dependent actin polymerization — and all this was in just the first wave of molecular analysis of ActA. Tilney has published other results on unusual properties of actin polymerization in a variety of species. I think it’s safe to predict that there might still be a few turning points ahead for cell biologists who read his papers, even if they don’t see comets.