Partitioning of ribonucleoprotein complexes from the cellular actin cortex

The cell cortex plays a crucial role in cell mechanics, signaling, and development. However, little is known about the influence of the cortical meshwork on the spatial distribution of cytoplasmic biomolecules. Here, we describe a new fluorescence microscopy method to infer the intracellular distribution of labeled biomolecules with sub-resolution accuracy. Unexpectedly, we find that RNA-binding proteins are partially excluded from the cytoplasmic volume adjacent to the plasma membrane that corresponds to the actin cortex. Complementary diffusion measurements of RNA-protein complexes suggest that a rudimentary model based on excluded volume interactions can explain this partitioning effect. Our results suggest the actin cortex meshwork may play a role in regulating the biomolecular content of the volume immediately adjacent to the plasma membrane.

Methods
Dual-color z-scans using two-photon microscopy and fluorescence correlation spectroscopy measurements of protein mobility. The DC z-scan data is processed using the supplied fitter (see IDL code examples) to generate estimates of the effective depletion length, which are stored in CSV format. The FCS data are fit to a 2D Gaussian model to extract the effective diffusion coefficient, which is also stored in CSV format.