No abstract

Interferon-induced transmembrane proteins (IFITMs 1, 2 and 3) have emerged as important innate immune effectors that prevent diverse virus infections in vertebrates. However, the cellular mechanisms and live-cell imaging of these small membrane proteins have been challenging to evaluate during viral entry of mammalian cells. Using CRISPR–Cas9-mediated IFITM-mutant cell lines, we demonstrate that human IFITM1, IFITM2 and IFITM3 act cooperatively and function in a dose-dependent fashion in interferon-stimulated cells. Through site-specific fluorophore tagging and live-cell imaging studies, we show that IFITM3 is on endocytic vesicles that fuse with incoming virus particles and enhances the trafficking of this pathogenic cargo to lysosomes. IFITM3 trafficking is specific to restricted viruses, requires S-palmitoylation and is abrogated with loss-of-function mutants. The site-specific protein labeling and live-cell imaging approaches described here should facilitate the functional analysis of host factors involved in pathogen restriction as well as their mechanisms of regulation.

Over the past years, fluorescent proteins (e.g., GFP) have been widely utilized to visualize recombinant protein expression and localization in live cells. Although powerful, fluorescent protein tags are limited by their relatively large size and potential perturbation to protein function. Alternatively, site-specific labeling of proteins with small-molecule organic fluorophores using bioorthogonal chemistry may provide a more precise and less perturbing method. This approach involves site-specific incorporation of unnatural amino acids (UAAs) into proteins via genetic code expansion, followed by bioorthogonal chemical labeling with small organic fluorophores in living cells. While this approach has been used to label extracellular proteins for live cell imaging studies, site-specific bioorthogonal labeling and fluorescence imaging of intracellular proteins in live cells is still challenging. Herein, we systematically evaluate site-specific incorporation of diastereomerically pure bioorthogonal UAAs bearing stained alkynes or alkenes into intracellular proteins for inverse–electron–demand Diels–Alder cycloaddition (IEDAC) reactions with tetrazine-functionalized fluorophores for live cell labeling and imaging in mammalian cells. Our studies show that site-specific incorporation of axial diastereomer of trans-cyclooct-2-ene-lysine (2’-aTCOK) robustly affords highly efficient and specific bioorthogonal labeling with monosubstituted tetrazine-fluorophores in live mammalian cells, which enabled us to image the intracellular localization and real-time dynamic trafficking of IFITM3, a small membrane-associated protein with only 137 amino acids, for the first time. Our optimized UAA incorporation and bioorthogonal labeling conditions also enabled efficient site-specific fluorescence labeling of other intracellular proteins for live cell imaging studies in mammalian cells.

Highlights d CHP1 regulates the major route of glycerolipid synthesis in mammals and invertebrates d CHP1 binds and activates GPAT4, the rate-limiting enzyme of ER glycerolipid synthesis d Activation of GPAT4 requires myristoylation of CHP1 d Loss of CHP1-GPAT4 complex leads to a dependency on peroxisomal lipid synthesis