The subcellular distribution of the human Fic protein, HYPE/FicD, using a high resolution, tomography compatible electron microscopic method

Abstract

Localization of membrane proteins via electron microscopy (EM) at high resolution is dependent on a robust detection technology and a sample preparation method that confers superior ultrastructural preservation of membranes. Current methods for precise localization rely on chemical fixation of the sample followed by alcohol dehydration at room temperature. The combination of these traditional methods exhibit poor preservation of membrane, induces membrane artifacts and affects resolution. High pressure freezing (HPF) of cells followed by heavy metal staining via freeze substitution (FS) in acetone at sub‐zero temperatures are techniques that exhibit the best possible membrane preservation. Therefore, we have developed a hybrid method that combines high pressure freezing with an existing peroxide based detection technique typically used with chemical fixation methods. As proof of concept, we assessed the localization of a fusion construct, consisting of the N‐terminus of HMG CO‐A protein fused with a peroxidase tag (that faces the cytosol). We exploited the induction of OSERs (organized smooth ER) in cells by the over‐expression of this N‐terminal fragment, as a metric for specific staining and membrane preservation. We next extended our studies to assess the localization of the sole human Fic (filament induced by cyclic AMP) protein, HYPE. We previously showed that HYPE resides in the ER lumen, but various global transcriptomic and proteomic studies suggest the existence of cytosolic and extracellular substrates for HYPE. Using our hybrid methodology, here we provide unequivocal evidence that the HYPE is compartmentalized within the various subdomains of the ER. We further demonstrate via serial sectioning, that our technique enables the localization of HYPE through a large volume of the cell. Our work thus aligns well with the three‐dimensional mapping of membrane proteins in large organelles using EM‐tomography in a near‐native cellular context.Support or Funding InformationThis work was supported by funding from 1) an Indiana CTSI (Clinical and Translational Sciences Institute) Project Development Team Award and 2) the National Institute of General Medical Sciences (NIGMS) of the National Institutes of Health (NIH) under Award Number R01GM100092.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.