Elucidating signal transduction in multi-domain BLUF photoreceptors by studying the Photoactivated Adenylate Cyclase OaPAC

Photosensory receptors, essential molecular entities across all domains of life, enable organisms to detect and respond to light stimuli, underpinning their critical involvement in regulating biological processes such as phototropism, circadian rhythms, photomorphogenesis, and photosynthesis. Among the myriad types of photosensory receptors, blue light sensing proteins such as Blue Light Using Flavin (BLUF) photoreceptors distinguish themselves through their ability to utilize blue light for signalling. Characterized by the conserved structure of their sensor domain, BLUF photoreceptors are found in a wide array of organisms, from bacteria and algae to plants and certain fungi. Known for their capacity to bind flavin chromophores, typically flavin adenine dinucleotide (FAD), they undergo conformational changes upon blue photon absorption, leading to downstream signalling events, highlighting their pivotal role in the adaptive responses of various organisms to light. This dissertation provides a comprehensive exploration of the BLUF photoreceptors, particularly focusing on the Photoactivated Adenylate Cyclase protein from Oscillatoria acuminata (OaPAC), which comprises a BLUF sensor domain linked to an Adenylate Cyclase (AC) effector domain, catalysing the conversion of ATP into cAMP. This study aims to elucidate the photoactivation mechanism of OaPAC and the ensuing signal transduction pathway, employing an integrative approach that leverages time-resolved crystallography, small angle X-ray scattering, spectroscopy, and biochemical characterization techniques. Special emphasis is placed on the Tyr-Gln-Met triad in the BLUF domain, which plays a crucial role in the initial light-induced rearrangements. Additionally, significant attention is given to the less understood aspects of BLUF photoreceptors, particularly the transduction of the initial light signal to more distal parts of the protein, which ultimately leads to biological activity. This research identifies a Met(out)/Trp(in) transition as a crucial element in conveying the signal to the α-helix linker region. Finally, structural models of OaPAC with ATP bound in the active site, along with complementary FTIR investigations, provide a thorough understanding of ATP binding and allosteric communication. As a result, the research presented in this dissertation not only expands the fundamental understanding of BLUF photoreceptor biology, but also provides a framework for future studies aimed at deciphering complete signal transduction pathways in multi-domain BLUF photoreceptors and towards the development of optogenetic tools.