In order to front-load the evolution of metazoan life forms, unicellular organisms would likely be endowed with a toolkit to stitch individual cells into sheets. We have previously seen that various protozoa, when take together, have most of the toolkit for forming adherens junctions. Yet another type of cell junction is the tight junction, commonly used in epithelial tissues.

As you can see from the figure below, tight junctions effectively sew cells together, allowing a sheet of cells to serve as a barrier to regulate the transport of material into and out of the underlying tissue. .

As you can also see from the figure, the backbone of the tight junction is composed of the membrane proteins occludin and claudin. Yet tight junctions are more sophisticated than a means of stitching cells together, as the membrane proteins interact with a set of linker proteins under the membrane which in turn interact with the cytoskeleton, allowing extracellular processes and forces to regulate cell behavior.

One such linker protein is known as MAGI. MAGI is composed of three domains. It contains multiple PDZ domains, which are common in signaling and scaffolding proteins. PDZ domains typically bind to the C-terminal domains of other proteins. MAGI also contains two WW domains, which typically function to bind proline-enriched regions of other proteins. Finally, MAGI also contains a GUK domain, that functions to convert GMP into GDP. It is this GUK domain which probably confers a regulatory function to MAGI as it bridges the membrane proteins and cytoskeleton.

While MAGI had previously been described in chordates and echinoderms, recently the protein was discovered in sponges (Teresa Adell,Vera Gamulin, Sanja Perovic-Ottstadt,Matthias Wiens,Michael Korzhev, Isabel M. Muller,and Werner E. G. Muller. 2004. Evolution of Metazoan Cell Junction Proteins: The Scaffold Protein MAGI and the Transmembrane Receptor Tetraspanin in the Demosponge Suberites domuncula. J Mol Evol 59:41–50). It is worth noting that sponge MAGI shares more sequence similarity and identity with the human sequence than it does with worm or fly sequence.

Another membrane protein, known as tetraspanin, very similar in architecture to the occludins and claudins, was also discovered. These findings led the researchers to conclude:

In summary, the identification of cell junction proteins, the tetraspanin receptor and the scaffold protein MAGI, confirms that during the evolutionary transition from the fungi, as the closest related kingdom to the Metazoa, to the multicellular animals, with the sponges as the oldest phylum, epithelial layers with cell junctions had been formed.

Might some of these components be found in unicellular life forms, adding to the toolkit for metazoan construction?