What is self-assembly? Why does it make sense that the flagellar filament
is assembled in this way?
Answers
Self-assembly is a process in which a disordered system of pre-existing components forms an organized structure or pattern as a consequence of specific, local interactions among the components themselves, without external direction.
The bacterial flagellum is a biological macromolecular nanomachine for locomotion. A membrane embedded molecular motor rotates a long helical filament that works as a propeller driving the bacterium through the liquid environment. The flagellum is composed of about 30 different proteins with copy numbers ranging from a few to a few thousands and is made by self-assembly of those proteins. The helical filament can be transformed into various distinct supercoiled forms by changes in chemical environment, single amino acid mutations, or mechanical forces. The axial portion of the flagellum involves several substructures: the rod, the hook, the hook-filament junction, the long helical filament and a cap at the filament tip. Although the axial component proteins are dissimilar at the primary sequence level, they share common structural characteristics. The central portions of their amino acid sequences are hypervariable whereas the amino- and carboxy-terminal parts contain highly conserved segments. In their monomeric form, they posses large, natively-disordered terminal regions that are essential in controlling and mediating intersubunit interactions. Recent structural studies—combining X-ray diffraction and single particle image analysis by cryo-EM—have given insights into the molecular mechanisms of self-assembly, supercoiling and polymorphic ability and open the way for construction of various flagella-based systems for applications in vaccination, bio- or nanotechnology.