The cell’s skeleton, or cytoskeleton, is involved in just about every major activity that cells perform, and the actin cytoskeleton in particular. This month’s Nature Cell Biology has a review on the actin ‘microfilaments’ and their roles in over 15 cell activities, that nicely underscores the diversity of functions one component can be co-opted for in metazoan cells. As Chhabra and Higgs describe in The many faces of actin: matching assembly factors with cellular structures:
… actin polymerizes into one type of structure in eukaryotic cells: helical filaments. However, these filaments can be assembled further into a wide variety of higher-order cellular structures ranging from lamellipodia to microvillia (Fig. 1), each of which performs specific functions. We ask one basic question in this review: how are filaments assembled for these structures?
Chhabra and Higgs take the proximate and reductionist approaches, and explain the detailed interaction networks which facilitate all of these structures, but there is one aspect that is missing: the evolutionary context in which this array of structures are assembled from one relatively simple actin filament.
There are a lot of terms that can be used to describe this phenomenon, and I’m not completely satisfied with any of the more popular terms, largely because they carry anthropomorphic connotations (e.g. preadaptation). As a term, exaptation works better, because the most parsimonious way of explaining such progressions is to explain that it is what comes before that determines what comes after, not the purpose of what comes after that dictates the process getting there.
Unfortunately, these terms were created by anatomists, and not by cell and molecular biologists, so we’re stuck using terms that weren’t meant to apply to cellular and cytoskeletal structures at all. Other times we’re using terms from paleontology, such as adaptive radiation – which it is in a sense, because protein functions are evolving in such a way within a genome (as opposed to an ecosystem), but it leaves out the role that modularity and duplications play in the evolution of gene/protein function.
Whatever the best way to describe their evolutionary past, the many faces of actin are indeed diversified.
- Chhabra ES, Higgs HN. The many faces of actin: matching assembly factors with cellular structures. Nat Cell Biol. 2007 Oct;9(10):1110-21. DOI:10.1038/ncb1007-1110