How a Flagellum Is Built: The Intraflagellar Transport (IFT)

The mechanisms that determine and preserve the size and function of cellular organelles represent a fundamental question in cell biology up to now only partially understood, and flagella has provided a handy model system to investigate organelles’ size-control analysis. It was discovered that flagella are dynamic structures and that flagellar length is regulated by a process called intraflagellar transport. IFT is a motile process within flagella in which large protein complexes move from one end of the flagellum to the other, and flagellar length is regulated by a balance between continuous assembly of tubulin at the tip of the flagellum, counterbalanced by continuous disassembly. According to Iomini et al. (2001), the IFT cycle consists of four phases. In Phase I, which takes place in the basal body region of the flagellum, anterograde particles are assembled from retrograde particles by remodeling or exchange of subunits with the cell body cytoplasm, with a concurrent decrease in number.

In this phase, the precursors of the flagellar structures that make up the cargos are also loaded onto the particles. In Phase II, the particles are transported from the base to the distal end of the flagellum by a heterometric kinesin II with a velocity of about 2 mm sec21. In Phase III, which occurs at the distal end of the flagellum, anterograde particles are remodeled into retrograde particles with a concurrent increase in number, probably upon or after unloading their cargo. Finally, in Phase IV, retrograde particles are transported by a cytoplasmic flagellar dynein from the distal end back to the basal body region of the flagellum, with a velocity of about 3 µm sec^-1, higher than that of anterograde particles.

How a Flagellar Motor Works

Movement can arise by shape change of permanently linked elements, by reversible interactions causing movement of elements relative to each other, by reversible assembly and disassembly, etc. all of which need energy input. We know that such changes can occur in proteins, the most likely molecules serving these locomotory functions in real movement system. But what drives and controls these changes? In principle, the problem is not difficult. Altering the ionic milieu, changing chemically or electrically its environment can alter the tertiary or quaternary structure of a protein. In most control systems, if not all, a change in the environment brings about a change in the properties of the motor, acting either directly or indirectly on the component of the motor. We need only two proteins to make a motor using the sliding filaments mechanism, that is, a globular protein (such as tubulin) and an anchor protein (such as the dynein–dynactin complex). If the globular protein can polymerize, we can assemble it into a linear polymer that can be attached via the anchor protein to another structure some distance away. The transformation of chemical energy into mechanical work depends on a conformational change of the anchor protein, which uses the hydrolysis of ATP into ADP.

Provided the anchor protein repeats the conformational change upon each monomer of the globular protein in turn, the “boat” can be hauled “hand over hand” towards the distant anchorage. Provided some kind of metachrony regulates adjacent motor molecules, we can link our small movements in a temporal series to amplify the amount of movement that can be achieved. Each step costs hydrolysis of one ATP molecule per anchor protein. The simplest and most obvious solution is either to have more than one anchor protein, or to have a dimer, working out-of-phase, being careful not to detach before the new attachment is formed. For instance, most (but not all) microtubular motors (dyneins, kinesins) work as dimers whose subunits walk along microtubule walls just like human legs walk on a surface. Once we have two hands to pull on the rope we can indeed move hand-over-hand; the onearmed man cannot do more than pull once. The flagellum movements are due to the transient interaction between two anchored microtubules, coupled to a linkage control. The generation of sliding of adjacent doublets by flagellar dynein is combined to the resisting forces localized near the active sliding rows of dyneins. During the cycle of binding/release obtained by dynein conformational change coupled to ATP hydrolysis, chemical energy is converted into mechanical energy used for sliding. Owing to their regular spacing every 24 nm along the axoneme, several adjacent dyneins participate to this local sliding and their functioning proceeds by local waves that propagate step by step all the way along the flagella. The postulated regulator has therefore to trigger the functioning of the different dyneins alternatively along the length as well as around the section of the axoneme, but its molecular nature remains unknown.

The model of Lindemann (1994) accounts for wave generation and propagation, regulated by geometrical constraints. This model, the so-called “geometric clutch,” is based on the way a cylinder with nine generatrix (the nine outer doublets of the axoneme) changes form when submitted to bending. In the zone of curvature, the doublets located outside the curvature are brought apart while the doublets located inside the curvature come closer to each other. This makes the corresponding dynein molecules efficient for sliding. In contrast, dynein molecules located outside the curvature are too far for binding to adjacent microtubules: no active sliding can occur in this zone. This model is consistent with ultrastructural data of electron microscopy. As above, the functioning of the axonemal mechanics needs transient connections, which could possibly be disrupted by intrinsic proteolytic activities due to the combined activities of a protease/ligase system.