Sunday, July 13, 2008


Choanoflagellates are single-celled eukaryotic organisms – like amoebae, yeasts, or slime molds, as opposed to prokaryotic organisms like bacteria. Eukaryotic cells are different from prokaryotic ones in having a variety of internal and external structures, including a cell nucleus that contains the organism's genome.

One distinctive feature of a choanoflagellate is its flagellum, a whiplike structure made up of microtubules, which a choanoflagellate uses to propel itself in water. A choanoflagellate rather resembles an animal sperm cell.

But the type of animal cell that a choanoflagellate most strongly resembles is called a choanocyte (meaning "collared cell") and is found in sponges. Modern sponges were, until quite recently, considered to be lineal descendants of the earliest type of multicellular animal (metazoan). (As discussed here and here, comb jellies may be descended from an even earlier metazoan.)

A metazoan is more than simply a collection of cells living together in a cohesive colony, such as slime molds and some choanoflagellates. The cells of a metazoan are of different types, and they communicate among themselves in order to effect whatever behavior the organism has evolved to sustain itself.

With that in mind, it is quite interesting that the genome of choanoflagellates contains genes for three proteins that are used ubiquitously in metazoa for intercellular communication:

New Evidence That Ancient Choanoflagellates' Form Evolutionary Link Between Single-celled And Multi-celled Organisms (7/1/08)
What do humans and single-celled choanoflagellates have in common? More than you'd think. New research into the choanoflagellate genome shows these ancient organisms have similar levels of proteins that cells in more complex organisms, including humans, use to communicate with each other.

According to a paper published in the Proceedings of the National Academies of Science, these findings help confirm choanoflagellates' role as an evolutionary link between single-celled and multi-celled organisms. They also contend that these insights into the organism's genome may mean that the proteins used to help cells communicate may have other roles as well. ...

Choanoflagellates, or at least their ancestors, have long been suspected as being the bridge between microorganisms with only one cell and metazoan, or multi-cellular organisms. There are many clues that lead to this conclusion, including the fact that choanoflagellates are similar to the individual cells in ocean sponges and unlike most other flagellates, they use their flagellate, or tail, to push themselves through water, rather than being pulled by it.

By analyzing the recently-sequenced choanoflagellate genome, the researchers discovered another similarity between choanoflagellates and most metazoans--their genetic code carries the markers of three types of molecules that cells use to achieve phospho-tyrosine signaling proteins.

The type of signaling in question here utilizes phosphorylation – the addition of a phosphate (PO4) group to a protein at one or more of its constituent amino acids. It is tyrosine phosphorylation when the the phosphate is attached to a tyrosine unit. (Tyrosine is one of the 20 kinds of amino acids that make up proteins.)

This process is much like reading, writing, and erasing a single bit of information in a computer memory. It is the proteins that perform these operations that are found to be shared by choanoflagellates and most metazoa.
Animals depend on tyrosine phosphorylation to conduct a number of important communications between their cells, including immune system responses, hormone system stimulation and other crucial functions. These phospho-tyrosine signaling pathways utilize a three-part system of molecular components to make these communications possible.

Tyrosine kinases (TyrK) 'write' messages between cells by adding phospho-tyrosine modifications, protein tyrosine phosphatases (PTP) are molecules that modify or 'erase' these modifications, and Src Homology 2 (SH2) molecules 'read' these modifications so the recipient cell gets the message.

What is intriguing is that all three of these signaling proteins are found in choanoflagellates in significant amounts. Although the proteins exist in other single-celled organisms, they aren't found together or in the same amounts as they are in choanoflagellates. This is the sort of thing that makes a researcher think, "Hmmm, that's strange. Wonder what's up with that?"
The researchers conclude that the presence of the full three-component signaling system may have played a role in the development of metazoan organisms whose cells could communicate with each other in complex ways.

It would be interesting to find out whether choanoflagellates actually use the proteins to communicate among themselves, and if so, for what purposes. But those are questions that remain to be answered.

Further reading:

The genome of the choanoflagellate Monosiga brevicollis and the origin of metazoans – an earlier (2/08) journal article on the choanoflagellate genome, and possible links to metazoa (sub. rqd. for full access)

The Premetazoan Ancestry of Cadherins – companion research article to the preceding, from Science, 2/15/08 (sub. rqd. for full access)

Genome Of Marine Organism Tells Of Humans' Unicellular Ancestors (2/14/08) – press release that describes the preceding research

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Anonymous Anonymous said...

Oddly enough there is a similar but more detailed analysis of the same pathways in another recent PNAS paper (Manning et al). From that it seems as though this mechanism is quite old, it's just that the tyrosine kinase part was done by related but distinct class of kinases, and that these were subsumed by 'modern' tyrosine kinases in both animals and choanoflagellates. So the fact that tyrosine kinases were invented a little earlier that animals is nothing new (it was covered in the genome paper and by King and Carroll several years ago), but the amazing thing is that the system is so elaborate in choanoflagellates, yet the evolutionary history of these genes is so distinct, maybe they have something to tell us. There's some more detail at the Kinase.Com portal.

7/14/2008 08:40:00 AM  

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