Imaging the structure of the plasma membrane with super-resolution light and electron microscopy
The plasma membrane separates the cell’s interior from the outside world. The exchange of information and material across this barrier is regulated by a multitude of channels, transporters, receptors, and trafficking organelles. Mapping the structure and dynamics of the plasma membrane is key to understanding how cells function. Electron microscopy can produce high resolution images of the structure of the plasma membrane. It has been challenging, however, to identify proteins within these samples. Super-resolution localization microscopy can image specific fluorescently-labeled molecules with better than 20 nm precision. We recently developed a correlative super-resolution light and platinum replica EM method (CLEM) that couples these complementary methods to produce images where identified proteins are mapped within the dense structural context of the cell at the nanoscale. This correlative method is uniquely suited to map the nanometer-scale molecular organization of the plasma membrane and associated organelles. Using this CLEM method, we studied 19 key proteins involved in clathrin-mediated endocytosis. Our data provide a comprehensive molecular architecture of endocytic vesicles. We discover that key endocytic proteins distribute into three distinct spatial zones: inside, outside, and at the edge of the clathrin coat in human cells. The presence and amount of many factors within these zones change during organelle maturation. We propose that the formation and curvature of single clathrin-coated vesicles is driven by the recruitment, re-organization, and loss of proteins within these three partitioned nanoscale zones during endocytosis.