Cell organelles must be in the right place at the right time to do their job. While some organelles are bound by membranes, others are not. Cells transport membrane-bound organelles like trucks on a highway. Membranes are important for transport since ‘trucks’ of specialized motor proteins dock to the organelle membranes and transport them as cargo along ‘highways’ of cytoskeletal tracks. But how then do cells transport the other class of organelles, which are liquid-like and lack a surrounding membrane? How do you position a water droplet, so to speak?

 

We can make water droplets by cooling water vapor, but it is quite difficult to see how this can be done specifically at one place or another. A simple way could be to heat one end of a closed chamber while cooling the other end. Water droplets evaporate from the hot end and condense at the cold end. We discovered that cells use a similar mechanism to position liquid-like ‘P granule’ organelles to one of its two poles, a process critical for fertility in the worm C. elegans. Signals from the cell-polarity system concentrate a regulator protein at one of the two poles of the C. elegans early embryos. This regulator protein dissolves P granules. The dissolved material at one pole then recondenses as P granules at the other pole where the regulator concentration is low.

 

While trying to understand how the regulator dissolves P granules, we discovered that a novel competition mechanism is at the heart of this problem. P granule proteins must bind to messenger RNA (mRNA) molecules to make P granules. It turns out that the regulator and the P granule proteins compete for these mRNA molecules, such that P granules dissolve when the regulator protein sequesters mRNA molecules away from the P granule proteins. This competition mechanism represents a powerful way, distinct from ‘trucks on a highway’-based transport, of organizing the spatial distribution of liquid-like RNA-protein organelles in cells.