Aardvark

One of the organisms that has a homlog to the Volvox version of beta-catenin is Dictylostium, otherwise known as a slime mold. This organism normally exists as single-celled amoeba, but when stressed, the cells seek each other out to form a multicellular state that looks like a slug, where all cells coordinate with each other for motility and reproductive purposes.

Below is a figure of the lifecycle of this organism:

As you can see, this organism can exist in an amoeboid state, a flagellated state, a multicellular state that mimics slugs and fungi, and as a spore. It even undergoes meiosis. All that biotic diversity packed into a single genome.

Here is a nice video that allows you to see this creature in action:

As mentioned before, the homologs to the Volvox version of beta-catenin are mostly annotated as hypothetical proteins because they were identified from DNA sequence rather than biochemical or genetic screens. Not so with Dictylostium. Its homolog is known. And has been studied. It’s called aardvark.

Remember that beta-catenins had a dual function. Not only did they play a crucial role in connecting a membrane protein to the cytoskeleton to form the adherens junctions used to connect cells, they also could travel to the nucleus to activate genes involved in growth and development. Well, feast your eyes on the function of aardvark:

Nature 408, 727-731 (7 December 2000)
Mark J. Grimson1,2, Juliet C. Coates2,3,4, Jonathan P. Reynolds3, Mark Shipman3, Richard L. Blanton1 & Adrian J. Harwood3

Adherens junctions and beta-catenin-mediated cell signalling in a non-metazoan organism

Mechanical forces between cells have a principal role in the organization of animal tissues. Adherens junctions are an important component of these tissues, connecting cells through their actin cytoskeleton and allowing the assembly of tensile structures1, 2, 3, 4. At least one adherens junction protein, beta-catenin, also acts as a signalling molecule, directly regulating gene expression5, 6, 7. To date, adherens junctions have only been detected in metazoa, and therefore we looked for them outside the animal kingdom to examine their evolutionary origins. The non-metazoan Dictyostelium discoideum forms a multicellular, differentiated structure8. Here we describe the discovery of actin-associated intercellular junctions in Dictyostelium. We have isolated a gene encoding a beta-catenin homologue, aardvark, which is a component of the junctional complex, and, independently, is required for cell signalling. Our discovery of adherens junctions outside the animal kingdom shows that the dual role of beta-catenin in cell–cell adhesion and cell signalling evolved before the origins of metazoa.

So what do we have here? Beta-catenin is a protein with two different crucial roles to play in metazoan existence: it is a key component of the adherens junction, thus epithelial tissue, and it is part of a key circuit that regulates gene expression needed for growth and development. In essence, beta-catenin integrates information from multiple sources: the genome, the cytoskeleton, the membrane, and the environment.

As you can now see, this integrator existed prior to the existence of metazoan life, where it plays same basic functions in an organism that exists most of the time in a single-celled state. What’s more, the integrator apparently has homologs in many distantly related eukaryotes, strongly suggesting it was part of the first truly eukaryotic cells. In other words, from the beginning of eukaryotic life, an important component of multicellular existence was already in existence.

So where did beta-catenin come from? Let’s look at that next.

[Added: Aardvark also works to help confirm the Volvox sequence is a beta-catenin homolog:  human beta-catenin sequence is more similar to Volvox sequence than Dictylostium sequence, yet the latter is an accepted homolog of human beta-catenin).

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