Allosterically regulated unfolding of the A'α helix exposes the dimerization site of the blue-light-sensing aureochrome-LOV domain.

Aureochromes have been shown to act as blue-light-regulated transcription factors in algae in the absence of phototropins. Aureochromes comprise a light-, oxygen-, or voltage-sensitive (LOV) domain as a sensory module binding the flavin chromophore and a basic region leucine zipper (bZIP) domain as an effector. The domain arrangement in aureochromes with an N-terminal effector is inversed to other LOV proteins. To clarify the role of the linking A'α helix in signaling, we have investigated the LOV domain of aureochrome1a from the diatom alga Phaeodactylum tricornutum without the N-terminal A'α helix but with the C-terminal Jα helix. Results were analyzed in comparison to those previously obtained on the LOV domain with both flanking helices and on the LOV domain with the A'α helix but without the Jα helix. Fourier transform infrared difference spectroscopy provides evidence by a band at 1656 cm(-1) that the A'α helix unfolds in response to light. This unfolding takes place only in the presence and as a consequence of the unfolding of the Jα helix, which points to an allosteric regulation. Size exclusion chromatography shows the LOV domain to be dimeric in the absence and monomeric in the presence of the A'α helix, implying that the folded helix covers the dimerization site. Therefore, the A'α helix directly modulates the oligomerization state of the LOV domain, whereas the Jα helix acts as an allosteric regulator. Both the allosteric control and the light-induced dimerization have not been observed in phototropin-LOV2 and point to a different signaling mechanism within the full-length proteins.