The Rhizoclosmatium globosum genome encodes three rhodopsin-guanylyl cyclases (RGCs), which are predicted to facilitate visual orientation of the fungal zoospores. Here, we show that RGC1 and RGC2 function as light-activated cyclases only upon heterodimerization with RGC3 (NeoR). RGC1/2 utilize conventional green or blue-light-sensitive rhodopsins (λmax = 550 and 480 nm, respectively), with short-lived signaling states, responsible for light-activation of the enzyme. The bistable NeoR is photoswitchable between a near-infrared-sensitive (NIR, λmax = 690 nm) highly fluorescent state (QF = 0.2) and a UV-sensitive non-fluorescent state, thereby modulating the activity by NIR pre-illumination. No other rhodopsin has been reported so far to be functional as a heterooligomer, or as having such a long wavelength absorption or high fluorescence yield. Site-specific mutagenesis and hybrid quantum mechanics/molecular mechanics simulations support the idea that the unusual photochemical properties result from the rigidity of the retinal chromophore and a unique counterion triad composed of two glutamic and one aspartic acids. These findings substantially expand our understanding of the natural potential and limitations of spectral tuning in rhodopsin photoreceptors.

2 These authors made an equal contribution. Basement membrane molecules such as laminin are important structural components of the skin 1-4 , but also serve as substrates for sensory neurons of the dorsal root ganglia (DRG) to grow in culture 5 . The main function of sensory neurons innervating the skin is to detect and relay relevant sensory stimuli, in particular mechanical stimuli 6 . It has long been known that sensory neurons with a nociceptive function (detecting potentially harmful stimuli) can have their endings in the epidermis 7-9 whereas mechanoreceptor endings (touch receptors) reside exclusively in the dermal layer [9][10] . Interestingly, the matrix environments of the epidermis and the dermis are very distinctive 11 . We showed that mechanosensitive currents required for touch receptor function depend on the presence of a protein tether which may function to couple mechanosensitive channels to a laminin-containing matrix 12 . The tether protein is not required for the mechanosensitivity of most nociceptive sensory neurons. Here we set out to address the idea that sensory mechanotransduction might be modulated by distinct matrix components made by different types of skin cells in different skin layers. We show that epidermal keratinocytes produce a matrix that is non-permissive for mechanotransduction and identify the factor responsible as laminin-332 (formerly known as laminin-5). Laminin matrices doped with small amounts of laminin-332 have a dramatically altered network structure that is non-permissive for tether attachment.We demonstrate a spatially restricted loss of mechanotransduction in neurite segments connected to laminin-332-containing matrices. Mutations in all three genes coding the trimeric laminin-332 protein complex can cause epidermolysis bullosa, a severe inherited skin blistering disease 1,3 . Human keratinocytes that produce a laminin-332 free matrix have no inhibitory activity on mechanotransduction. We have also discovered an activity of laminin-332 matrix in inhibiting sensory axon bifurcation. Our results reveal novel mechanisms whereby permissive and non-permissive substrates can spatially coordinate mechanotransduction in distinct domains within a single neuron. Results Keratinocyte matrix is suppresses mechanotransductionUsing whole-cell, patch-clamp techniques we directly recorded mechanosensitive currents in cultured sensory neurons [12][13][14][15][16][17][18][19][20] . We first asked whether co-culture of sensory neurons with different cellular components of the skin can modulate the activity of mechanosensitive currents. When sensory neurons are cultured on a laminin substrate, standardly-derived from Engelbreth-Holm-Swarm cells (EHS matrix, henceforth referred to as laminin), more than 90% of the cells exhibit a mechanosensitive current evoked using a small (~740 nm displacement) stimulus to the neurite 12, 14-15 . At least three types of mechanosensitive current can be measured in sensory neurons, classified according to their inactivation time constant τ 1 , rap...

Optogenetics enables manipulation of biological processes with light at high spatio-temporal resolution to control the behavior of cells, networks, or even whole animals. In contrast to the performance of excitatory rhodopsins, the effectiveness of inhibitory optogenetic tools is still insufficient. Here we report a two-component optical silencer system comprising photoactivated adenylyl cyclases (PACs) and the small cyclic nucleotide-gated potassium channel SthK. Activation of this ‘PAC-K’ silencer by brief pulses of low-intensity blue light causes robust and reversible silencing of cardiomyocyte excitation and neuronal firing. In vivo expression of PAC-K in mouse and zebrafish neurons is well tolerated, where blue light inhibits neuronal activity and blocks motor responses. In combination with red-light absorbing channelrhodopsins, the distinct action spectra of PACs allow independent bimodal control of neuronal activity. PAC-K represents a reliable optogenetic silencer with intrinsic amplification for sustained potassium-mediated hyperpolarization, conferring high operational light sensitivity to the cells of interest.

A new microbial rhodopsin class that actively transports sodium out of the cell upon illumination was described in 2013. However, poor membrane targeting of the first-identified sodium pump KR2 in mammalian cells has hindered the direct electrical investigation of its transport mechanism and optogenetic application to date. Accordingly, we designed enhanced KR2 (eKR2), which exhibits improved membrane targeting and higher photocurrents in mammalian cells to facilitate molecular characterization and future optogenetic applications. Our selectivity measurements revealed that stationary photocurrents are primarily carried by sodium, whereas protons only play a minor role, if any. Combining laser-induced photocurrent and absorption measurements, we found that spectral changes were not necessarily related to changes in transport activity. Finally, we showed that eKR2 can be expressed in cultured hippocampal mouse neurons and induce reversible inhibition of action potential firing with millisecond precision upon illumination with moderate green-light. Hence, the light-driven sodium pump eKR2 is a reliable inhibitory optogenetic tool applicable to situations in which the proton and chloride gradients should not be altered.