Many dynein teams collectively generate high forces during the transport of large organelles

The transport of organelles is important to maintain cellular organization and function. Efficient retrograde transport of large organelles with a size of several micrometers requires high collective forces from multiple dynein motors. However, the exact transport forces and their dependence on the cargo size are unknown for large organelles. Furthermore, it is not known how many dynein motors are active during this transport and how they generate high collective forces sufficient to overcome the cytoplasmic drag. We measured forces generated during retrograde transport of phagosomes with diameters between 1 and 5 μm. Forces increased with phagosome volume and ranged from under 10 pN for the smallest up to 160 pN for the largest phagosomes. These forces matched the cytoplasmic drag to achieve equally fast transport with a velocity of 25 ± 4 nm s^-1 for phagosomes of all sizes. To confirm the need for many dynein motors to generate such high forces, we labeled and quantified dynein on isolated phagosomes. We found up to 250 dyneins on the largest phagosomes and a dynein surface density that was independent of the phagosome size. We connected the dynein numbers and transport forces with a theoretical model of the microtubule distribution around the organelles. The model implies that, because larger organelles displace and bend the microtubules, disproportionately large numbers of dyneins can be active and contribute to the high transport forces of large phagosomes. Our results indicate that, during the transport of large organelles, many dyneins interact with multiple microtubules in a cargo-size-dependent manner to achieve sufficiently large transport forces.