Structural Biology of Macromolecular Complexes
Large multi-subunit complexes play an important role in biology. These complexes are often the intracellular machines that perform diverse functions such as synthesis (such as the ribosome), vesicle trafficking (such as clathrin and COPII), and transport (such as dynein). One powerful method for studying how these complexes perform their functions is cryo-electron microscopy (cryoEM). In cryoEM, images are taken of specimens that have been embedded in a thin layer of vitreous ice, thus they are close to their native environment. Then, using techniques like tomography or single particle reconstruction, the specimens are reconstructed in three-dimensions. This 3-D picture can give valuable insights into the mechanisms that biological molecules employ to perform their functions.
In our lab, we use cryoEM to study the structure of coat protein complex II (COPII) coated vesicles. In exocytosis and endocytosis, vesicular carriers mediate the transport of protein and lipid between various cellular compartments. These are fundamental pathways in eukaryotes, and their disruption can result in cell death or one of several diseases. COPII coated vesicles mediate the transport of cargo from the endoplasmic reticulum (ER) to the Golgi apparatus (Fig. 1). COPII vesicles are assembled from the association of three subunits: Sar1, Sec23/24, and Sec13/31. We have solved the structure of the COPII cage, assembled from Sec13/31 heterotetramers (Fig. 2), and the COPII coat, assembled from both Sec13/31 and Sec23/24. These structures revealed several novel insights into the formation of COPII coats in vivo and the mechanisms by which the coat adapts to accommodate cargo of widely varying sizes and shapes. Current studies in the lab are aimed at increasing the resolution of the 3-D reconstructions of COPII cages and COPII coats, measuring the flexibility of Sec13/31 cages as observed in cryoEM images, and reconstructing natively assembled COPII coated vesicles.
