Cytoskeleton

Abstract

As in multicellular organisms, single cells are confronted with challenges associated with structural support and delivery of biomolecules, albeit at a different scale. Most cells rely on endoskeletons (filaments and tubules) and/or exoskeletons (cell walls) to maintain cell-shape integrity. Cell division also requires membrane-deforming proteins. In eukaryotes, various modes of internal cellular movement require cytoskeletal highways for molecular motors, which transport large cargoes using ATP hydrolysis as fuel. Central to all of these cellular features are protein fibrils and sheets comprised of long concatenations of monomeric subunits held together by noncovalent forces. How fundamental features such as cell division were carried out prior to the origin of filament-forming proteins is unknown, but the emergence of self-assembling fibrils would have been a watershed moment for evolution, providing new opportunities for cellular features requiring structural support systems. This chapter continues an exploration of the internal anatomy and natural history of cellular components, exploring the evolutionary diversification of cytoskeletal proteins and their varied functions. The diverse sets of eukaryote-specific molecular motors and their roles in intracellular transport will also be explored. Although prokaryotes are devoid of such machines, fibrillar proteins do exist in prokaryotes, playing central but contrasting roles in structural support and cell division relative to what is seen in eukaryotes. In eukaryotes, fibrillar proteins are also central to swimming and crawling, so this chapter will explore a few generalities with respect to cellular locomotion. Both prokaryotes and eukaryotes use flagella to swim, and such structures are sometimes suggested to be so complex as to defy an origin by normal evolutionary processes. However, not only are there plausible routes for the emergence of flagella via modifications of pre-existing cellular features, but flagella have evolved more than once. Notably, prokaryotic and eukaryotic flagella evolved independently and operate in completely different manners. Nonetheless, despite these differences, the limits to motility of single-celled organisms will be shown to follow some general scaling laws across the Tree of Life.