Visualization of Membrane Sorting and Fusion in Living Cells using Total Internal Reflection (TIR) and Multicolor Video Microscopy

Abstract

Post-Golgi sorting of different classes of newly synthesized proteins and lipids is central to the generation and maintenance of cellular polarity. to directly visualize the dynamics and location of apical/basolateral sorting and trafficking we used fast time-lapse multicolor video microscopy in living cells. Specifically, green fluorescent protein color variants (cyan, CFP; yellow, YFP) of apical cargo (GPI-anchored) and basolateral cargo (vesicular stomatitis virus glycoprotein, VSVG) were generated; see FIG 1. Fast dual color fluorescence video microscopy allowed visualization with high temporal and spatial resolution. Our studies revealed that apical and basolateral cargo progressively segregated into large domains in Golgi/TGN structures, excluded resident proteins, and exited in separate transport containers. These carries remained distinct and did not merge with endocytic structures en route to the plasma membrane. Interestingly, our data suggest that the primary sorting occurs by lateral segregation in the Golgi, prior to budding (FIG 2). Further characterization of morphological differences of apical versus basolateral transport carriers was achieved using a specialized microscopy technique called total internal reflection (TIR) microscopy. with this approach only the bottom of the cell (<100 nm) was illuminated by an exponentially decaying evanescent “wave” of light. A series of images, taken at ∼1 second intervals, shows a bright “flash” of fluorescence when the vesicle fuse with the plasma membrane and the fluorophore diffuses into the plasma membrane (FIG 3).