ER-Organelle Contact Sites
Traditionally, the ER is thought of as the organelle responsible for lipid synthesis, protein synthesis, and calcium storage. However, our understanding of the cellular role for the ER has expanded a great deal with the characterization of membrane contact sites (MCSs). MCSs are sites where the ER comes in close apposition (less than 30nm) to another organelle membrane forming a contact site where lipid exchange, calcium signaling, and even enzymatic activity can occur. We use time-lapse live confocal fluoresence microscopy to image MCSs between thedynamic ER network and other organelles in mammalian cells. We have determined that all mitochondria and most endosomes are tightly associated with the tubular ER network, and remain so even as they traffic along microtubules. Thus, these organelles are trafficking around the cytoplasm pulling the ER tubular network with them. The stability and prevalence of these contacts suggest that bound organelles rely on ER contact for some aspect of their biogenesis.
Cos-7 cell expressing BFP-KDEL (green) and mCh-Rab7 (red). An endsome will extend and grow out, preparing for fission, until a newly formed ER tubule grows and contacts the bud at the site of constriction and division. White arrow marks position of endosome fission.
Cos-7 cell expressing GFP-Sec61ß (green), mCh-Drp1 (red), and mito-BFP (blue). A Drp1 puncta assembles at the site of an ER tubule crossing and mitochondrial constriction. Then division occurs, the Drp1 puncta and ER tiubule split and remain with both halves of the divided mitochondria .
Our ability to simultaneously image the coordinated dynamics of multiple organelles led us to make the surprising discovery that the ER is a conserved player in mitochondrial division. ER MCSs mark the position where the division machinery is recruited from the cytoplasm to the mitochondrial membrane. By high-resolution EM tomography, we can see that smooth ER tubules circumscribe and contact mitochondria at the position of constriction. Additionally, it seems the mechanisms of ER mediated division is conserved for endosomes. Stable contacts form between the ER and endosomes and the site of constriction and fission for early and late endosomes is spatially and temporally linked to ER contact sites. Disruption of ER structure and dynamics specifically and dramatically decreases the frequency of endosome fission. These new phenomena are just now beginning to be investigated and we are working to unravel the mechanisms that establish and maintain ER-organelle contacts, as well as identifying how the ER functions at MCSs.
Electron tomography reveals the three-dimensional structure of MCSs (coloured red) between ER tubules (green) and mitochondria (purple) in a yeast cell (right) or an endosome (yellow) in an animal cell (left). EM Tomography done by Matthew West.