Abstract
The modularity of light, oxygen, voltage (LOV) blue-light photoreceptors has recently been exploited for the design of LOV-based optogenetic tools, which allow the light-dependent control of biological functions. For the understanding of LOV sensory function and hence the optimal design of LOV-based optogentic tools it is essential to gain an in depth atomic-level understanding of the underlying photoactivation and intramolecular signal-relay mechanisms. To address this question we performed molecular dynamics simulations on both the dark- and light-adapted state of PpSB1-LOV, a short dimeric bacterial LOV-photoreceptor protein, recently crystallized under constant illumination. While LOV dimers remained globally stable during the light-state simulation with regard to the Jα coiled-coil, distinct conformational changes for a glutamine in the vicinity of the FMN chromophore are observed. In contrast, multiple Jα-helix conformations are sampled in the dark-state. These changes coincide with a displacement of the Iβ and Hβ strands relative to the light-state structure and result in a correlated rotation of both LOV core domains in the dimer. These global changes are most likely initiated by the reorientation of the conserved glutamine Q116, whose side chain flips between the Aβ (dark state) and Hβ strand (light state), while maintaining two potential hydrogen bonds to FMN-N5 and FMN-O4, respectively. This local Q116-FMN reorientation impacts on an inter-subunit salt-bridge (K117-E96), which is stabilized in the light state, hence accounting for the observed decreased mobility. Based on these findings we propose an alternative mechanism for dimeric LOV photoactivation and intramolecular signal-relay, assigning a distinct structural role for the conserved “flipping” glutamine. The proposed mechanism is discussed in light of universal applicability and its implications for the understanding of LOV-based optogenetic tools.
Highlights
Blue-light photoreceptors containing light-oxygen-voltage (LOV) domains regulate a variety of different physiological responses in both eukaryotes and prokaryotes (Demarsy and Fankhauser, 2009; Krauss et al, 2009; Herrou and Crosson, 2011)
The question as to how the light signal is relayed from the site of photon capture in the LOV domain active site to N- and C-terminal helical linker elements and to fused effector domains in full-length oligomeric multi-domain LOV photoreceptors remains, despite extensive experimental efforts, still largely unresolved. This is due to the fact that all photoexcited state X-ray structures, i.e., for LOV proteins crystallized in the dark and illuminated immediately before data collection, show only small structural changes compared to the corresponding dark-state structures (Crosson and Moffat, 2002; Fedorov et al, 2003; Halavaty and Moffat, 2007; Möglich and Moffat, 2007; Zoltowski et al, 2007; Endres et al, 2015)
Our simulations stress the importance of the conserved LOV domain glutamine for the overall signaling process and identify new key residues which might be involved in the signal relay from the site of photon capture to Nand C-terminally located effector domains via the A’α and Jα helical connectors
Summary
Blue-light photoreceptors containing light-oxygen-voltage (LOV) domains regulate a variety of different physiological responses in both eukaryotes and prokaryotes (Demarsy and Fankhauser, 2009; Krauss et al, 2009; Herrou and Crosson, 2011). Those in turn influence a multitude of different cellular light responses in plants (Möglich et al, 2010), bacteria (Herrou and Crosson, 2011), and fungi (Idnurm et al, 2010) In recent years, it became apparent, that adduct formation leads to small-scale structural changes in the vicinity of the FMN chromophore, which are in many cases relayed to the fused effector domains via helical interdomain linkers (termed N-terminal cap or A’α-helix and C-terminal Jα-helix) (Harper et al, 2003, 2004; Halavaty and Moffat, 2007, 2013; Nash et al, 2011; Diensthuber et al, 2013; Herman et al, 2013; Endres et al, 2015; Herman and Kottke, 2015). There is growing experimental evidence that those structural changes in turn result in altered LOV photoreceptor biological activities (Harper et al, 2004; Vaidya et al, 2011; Aihara et al, 2012; Okajima et al, 2014; Kashojiya et al, 2015)
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