Amoebae Get Organized
By RICHARD P. GRANT - THE SCIENTIST
Added: Wed, 07 Sep 2011 16:36:58 UTC
Dictyostelium discoideum is a social amoeba: a single-celled organism that can group together to form a multicellular slug. Although it has mostly been used as an experimental model for studying chemotaxis, cell-cell communication, and the evolution of sociality, some researchers are using it as a window into how multicellular life could have evolved. Now, Daniel Dickinson, a graduate student at Stanford University, has discovered that the Dictyostelium slug does more than simply take on a multicellular form; it also creates a tissue that has only ever been seen before in animals.
When Dickinson became a student of William Weis and James Nelson, who collaborate to study the adhesion protein α-catenin—a membrane protein necessary for forming epithelial tissues in animals—he wanted to explore how α-catenin had changed throughout evolution. He searched for α-catenin in sequence databases, and to his surprise, found the protein expressed in Dictyostelium. This protein “had no business being present in any organism outside of animals,” says Dickinson, so he set out to find out what it was doing there.
Dickinson cloned α-catenin and raised antibodies against the protein. He ran Western blots of Dictyostelium at different stages in its life cycle, and found that α-catenin was only expressed in the multicellular stage. After experimenting with different ways of fixing and staining the cells, he found the protein localized to the cell-cell contacts in a single layer of cells at the tip of the fruiting body. Intriguingly, this layer of cells was polarized, arranging its Golgi, centrosomes, and proteins at either the top or the bottom of the cell—the first instance of polarized epithelial cells reported in Dictyostelium.
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