In many cells AF appear to comprise populations of short (1-2 μm length), with random or anti-parallel filament polarity, and not an obvious system of tracks for directed transport 5, 6. Meanwhile, with some exceptions in which AFs form uniformly polarised arrays, e.g., lamellipodia, filopodia and dendritic spines, AF architecture appears much more complex. This model makes intuitive sense as MTs in many cultured animal cells form a polarised radial network of tracks spanning >10 μm from the centrally located centrosome to the periphery and appear ideally distributed for long-range transport.
Meanwhile AFs (local roads) and myosin motors work downstream picking up cargo at the periphery, and transporting it for the ‘last μm’ to its final destination. This ‘highways and local roads’ model suggests that MTs are tracks for long-range transport (highways) between the cell centre and periphery, driven by kinesin and dynein motors.
In animal cells, unlike plants and yeast, microtubules (MTs) and actin filaments (AFs) are thought to regulate transport in a manner akin to the infrastructure of a developed nation 1, 2, 3, 4. Based on this, we suggest a model in which organelles and force generators (motors and track assemblers) are linked, forming an organelle-based, cell-wide network that allows their collective activity to rapidly disperse the population of organelles long-distance throughout the cytoplasm. Thus, in addition to melanophilin/myosin-Va, Rab27a can recruit SPIREs to melanosomes, thereby integrating motor and track assembly activity at the organelle membrane. Here, we show that the SPIRE-type actin nucleators (predominantly SPIRE1) are Rab27a effectors that co-operate with formin-1 to generate actin tracks required for myosin-Va-dependent transport in melanocytes. This suggests that in animals, as in yeast and plants, myosin/actin can drive long-range transport. On the contrary, using melanosomes of melanocytes as a model, we recently discovered that the motor protein myosin-Va works with dynamic actin tracks to drive long-range organelle dispersion in opposition to microtubules.
Nature Communications volume 11, Article number: 3495 ( 2020)Ĭell biologists generally consider that microtubules and actin play complementary roles in long- and short-distance transport in animal cells. Rab27a co-ordinates actin-dependent transport by controlling organelle-associated motors and track assembly proteins