Genome of marine organism tells of humans' unicellular...(oteins essential to cell-to-cell signali...)

"In animals, some of these proteins, called cadherins, evolved for linking cells together; they are the glue that prevents clumps of cells from falling apart," King said. "Choanoflagellates show no hint of multicellularity, but they have 23 genes for cadherin proteins, about the same as the fruit fly or the mouse."

In the Science paper, King and graduate student Monika Abedin report that some of these proteins are found around the base of the choanoflagellate cell, where the choanoflagellate attaches to surfaces, and around the tentacles, where bacteria are captured and ingested.

Perhaps, they argue, the last single-celled ancestor of all animals (including humans) employed these ancient cadherin proteins to bind and eat bacteria, while more complex metazoans adopted these proteins for gluing cells into a larger, many-celled creature. "The transition to multicellularity likely rested upon the co-option of diverse transmembrane and secreted proteins to new functions in intercellular signaling and adhesion," they wrote in Science.

"Choanoflagellates really are a unique window back in time to the origin of animals and humans. They are our best way of triangulating on that last unicellular ancestor of animals, because the fossil record is not there," said Dan Rokhsar, UC Berkeley professor of molecular and cell biology and program head for computational genomics at JGI. King and Rokhsar also are members of UC Berkeley's Center for Integrative Genomics.

Choanoflagellates are found abundantly in salt and fresh water around the world, where they gorge on bacteria. At about 10 microns across, they're about the size of another one-celled eukaryote, yeast. While yeast are well known to genetics researchers, however, c
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