A phytochrome sensory domain permits receptor activation by red light

Optogenetics and photopharmacology enable the spatio-temporal control of cell and animal behavior by light. Although red light offers deep-tissue penetration and minimal phototoxicity, very few red-light-sensitive optogenetic methods are currently available. We have now developed a red-light-induced homodimerization domain. We first showed that an optimized sensory domain of the cyanobacterial phytochrome 1 can be expressed robustly and without cytotoxicity in human cells. We then applied this domain to induce the dimerization of two receptor tyrosine kinases—the fibroblast growth factor receptor 1 and the neurotrophin receptor trkB. This new optogenetic method was then used to activate the MAPK/ERK pathway non-invasively in mammalian tissue and in multicolor cell-signaling experiments. The light-controlled dimerizer and red-light-activated receptor tyrosine kinases will prove useful to regulate a variety of cellular processes with light. Go deep with red: The sensory domain (S) of the cyanobacterial phytochrome 1 (CPH1) was repurposed to induce the homodimerization of proteins in living cells by red light. By using this domain, light-activated protein kinases were engineered that can be activated orthogonally from many fluorescent proteins and through mammalian tissue. Pr/Pfr=red-/far-red-absorbing state of CPH1.