Anomalous diffusion of TePixD and identification of the photoreaction product

TePixD is a blue-light sensor protein from the thermophilic cyanobacterium Thermosynechococcus elongatus BP-1 (TePixD Tll0078). Although the photochemistry has been examined, so far the photoproduct remains unknown. We have measured the diffusion coefficient (D) of TePixD in the dark by dynamic light scattering and have discovered a very peculiar diffusion property; the decamer oligomer has a larger D than that of the pentamer. Furthermore, D of the pentamer was found to be very close to that of the TePixD decamer photoreaction product. In order to investigate this reaction further, elution profiles of size-exclusion chromatography were measured under dark and illuminated conditions at low (40 μM) and high (1.1 mM) TePixD concentrations. On the basis of these results, we have concluded that the main photoreaction of the TePixD decamer is the dissociation into the pentamer. The secondary structure change associated with this reaction was found to be minor according to circular dichroism analysis.