General cell culture and transfection

HEK293T cells (ATCC, CRL-11268) were maintained at 37 °C in 5% CO₂ atmosphere in DMEM High Glucose with 4.5 mM L-glutamine (GE Life Sciences, SH30003.03) supplemented with 1 mM sodium pyruvate (Alfa, A11148), 10% fetal bovine serum (Sigma, F0926), and 1% penicillin-streptomycin (Corning, 30-002-CI). For a 96-well format used in all imaging-based experiments, 100 µL of media was used for culturing, whereas for a 12-well plate used for western blot or biochemical assays, 1 mL of media was used. Cells were monitored every 3 months to confirm the absence of mycoplasma contamination (Genlantis, MY01100). Cells were transiently transfected with LPEI (Polysciences, 23966) using a 0.5 mg/mL solution at a 5:1 LPEI:DNA (w/w) ratio in antibiotic-free DMEM. Following overnight transfection, the media was removed and the cells were gently washed once with regular DMEM to remove any excess unnatural amino acid. Cells were serum-starved (DMEM with 0.1% FBS, without antibiotics) for 4 h. The media was replaced with 90 µL of LCIS immediately prior to irradiation. Irradiation was performed on a UV transilluminator (VWR Dual Transilluminator at 365 nm) for two minutes (unless specified otherwise).