Velocities of Microtubule-Based Motors in Living Chlamydomonas

Abstract

Reports in the published literature suggest that the velocities of vesicle transport in living neurons are discrete and quantal (multiples of a fundamental velocity), with the instantaneous velocity being dependent upon the number of molecular motors driving transport (Shtridelman et al., Cell Biochem. Biophys, 2008). We similarly observed discrete changes in the velocity of microspheres undergoing saltatory transport on the flagella of Chlamydomonas, and that these velocities appeared to be dependent upon location along the flagellum. We therefore studied the movements of adherent microspheres on flagella, driven by the intracellular motors kinesin-2 and dynein-2, to determine whether transport is driven at multiple, discrete velocities and whether they are spatially correlated. We measured separately the translational velocities of unconstrained and optically trapped microspheres as a function of position along the flagellum. The velocities of unconstrained microspheres were on average about two-fold higher than those of trapped microspheres. Unconstrained microsphere velocities in the anterograde and retrograde directions were not spatially correlated except at turn-around points near the beginning and end of the flagellum where velocities were consistently lower. Histograms of these data showed a broad distribution of velocities and suggested no strong evidence for quantized velocities. For trapped microspheres, for a given anterograde or retrograde transport event we often saw at least two discrete velocities; however, any two transport events can have different 'slow’and ‘fast’velocities. Thus even when velocities are measured from a single microsphere at a specific position on a flagellum, combining multiple velocity histograms results in an apparently non-quantized, broad distribution of velocities. What causes the abrupt change between discrete velocities during movements of a microsphere is yet unknown.