Dissection of molecular assembly dynamics by tracking orientation and position of single molecules in live cells

Regulation of order, such as orientation and conformation, drives the function of most molecular assemblies in living cells, yet remains difficult to measure accurately through space and time. We built an instantaneous fluorescence polarization microscope, which simultaneously images position and orientation of fluorophores in living cells with single-molecule sensitivity and a time resolution of 100ms. We developed image acquisition and analysis methods to track single particles that interact with higher-order assemblies of molecules. We tracked the fluctuations in position and orientation of molecules from the level of an ensemble of fluorophores down to single fluorophores. We tested our system in vitro using fluorescently labeled DNA and F-actin in which the ensemble orientation of polarized fluorescence is known. We then tracked the orientation of sparsely labeled F-actin network at the leading edge of migrating human keratinocytes, revealing the anisotropic distribution of actin filaments relative to the local retrograde flow of the F-actin network. Additionally, we analyzed the position and orientation of septin-GFP molecules incorporated in septin bundles in growing hyphae of a filamentous fungus. Our data indicate that septin-GFP molecules undergo positional fluctuations within, ∼350nm of the binding site and angular fluctuations within ∼30° of the central orientation of the bundle. By reporting position and orientation of molecules while they form dynamic higher-order structures, our approach can provide new insights into how micron-scale ordered assemblies emerge from nanoscale molecules in living cells.Significance StatementIn living cells, the 3D architecture of molecular assemblies such as chromosomes, lipid bilayers, and the cytoskeleton is regulated through the interaction among their component molecules. Monitoring the position and orientation of constituent molecules is important for understanding the mechanisms that govern the structure and function of these assemblies. We have developed an instantaneous fluorescence polarization microscope to track the position and orientation of fluorescently labeled particles, including single molecules, which form micron-scale macromolecular assemblies in living cells. Our imaging approach is broadly applicable to the study of dynamic molecular interactions that underpin the function of micron-scale assemblies in living cells.