Evolution of the cadherin adhesome
When confronted with the immense complexity of the cadherin adhesome it is only natural to wonder how did this complexity arise? Furthermore, if one assumes that such a large network of proteins is essential for the function of cell-cell adhesion and that cell-cell adhesion was essential for the emergence of multicellularity but not needed before hand, then how did this major step in evolution occur?
Remarkably, thanks to the whole genome sequencing of many organisms we now know that the step from unicellular to multicellular actually occurred separately multiple times throughout evolution! (As reviewed in Abedin and King, Trends in Cell Biology Volume 20, Issue 12, 2010, Pages 734–742)
Taking advantage of the abundance of sequenced genomes, in a close collaboration with Paul Murray and Barry Honig (Columbia U. NY, NY), we are exploring the evolution of the cadherin adhesome, from our unicellular origins. It is a fascinating story because you can find a unicellular choanoflagellate that has cadherin but no catenins and an even more ancient slime mold that has catenins but no cadherin. How did they all come together in sea sponges is something we are now figuring out.
UPDATE (Nov 2014):
Paul’s work has now been published in a paper in Biology Open (Biol Open. 2014 Nov 13. pii: BIO20149761. doi: 10.1242/bio.20149761). One of the important findings in his paper is that most of the cadhesome components were already found in pre-metazoan unicellular organisms. Those organisms also had cadherin, but it’s tail didn’t have the capability to bind catenins. So, the switch from unicellular to metazoa didn’t involve the “invention” of many new proteins, but rather a few mutations in the cytoplasmic tail of cadherin that would allow it to bind catenins. The figure below (taken from the paper) demonstrates the early evolutionary origin of the cadhesome.