Mutational Analysis of Phototropin 1 Provides Insights into the Mechanism Underlying LOV2 Signal Transmission

Abstract

Phototropins (phot1 and phot2) are blue light-activated serine/threonine protein kinases that elicit a variety of photoresponses in plants. Light sensing by the phototropins is mediated by two flavin mononucleotide (FMN)-binding domains, designated LOV1 and LOV2, located in the N-terminal region of the protein. Exposure to light results in the formation of a covalent adduct between the FMN chromophore and a conserved cysteine residue within the LOV domain. LOV2 photoexcitation is essential for phot1 function in Arabidopsis and is necessary to activate phot1 kinase activity through light-induced structural changes within a conserved ␣-helix situated C-terminal to LOV2. Here we have used site-directed mutagenesis to identify further amino acid residues that are important for phot1 activation by light. Mutagenesis of bacterially expressed LOV2 and full-length phot1 expressed in insect cells indicates that perturbation of the conserved salt bridge on the surface of LOV2 does not play a role in receptor activation. However, mutation of a conserved glutamine residue (Gln 575 ) within LOV2, reported previously to be required to propagate structural changes at the LOV2 surface, attenuates light-induced autophosphorylation of phot1 expressed in insect cells without compromising FMN binding. These findings, in combination with double mutant analyses, indicate that Gln 575 plays an important role in coupling light-driven cysteinyl adduct formation from within LOV2 to structural changes at the LOV2 surface that lead to activation of the C-terminal kinase domain.Light is critical in shaping the growth and development of plants. The effect of light on plant morphogenesis is mediated through a variety of photoreceptors with specific spectral properties. Genetic analysis using the model plant Arabidopsis thaliana has shown that the effects of blue (390 -500 nm) and UV-A light (320 -390 nm) are mediated by at least two classes of blue light receptors, cryptochromes and phototropins (1-4). Phototropins function to regulate a range of photoresponses, including phototropism, stomatal opening, and chloroplast movement, all of which serve to optimize the photosynthetic efficiency of plants (2). Phototropin activity has also been implicated in regulating extension-growth responses in Arabidopsis such as cotyledon expansion (5), leaf expansion (6), and growth promotion under weak light conditions (7).Arabidopsis contains two phototropins designated phot1 and phot2 (2). Phot1 and phot2 are flavoprotein photoreceptors whose protein structure can be divided into two segments as follows: a photosensory domain at the N terminus and a serine/ threonine kinase domain at the C terminus (Fig. 1A). The N-terminal photosensory domain of the phototropins contains a repeated motif of ϳ110 amino acids called LOV1 and LOV2, respectively (8 -10). LOV 2 domains are members of the large and diverse superfamily of Per, Arnt, Sim (PAS) domains associated with cofactor binding and mediating protein interactions (11). LOV domains are more closely related to...

Phototropin Receptor Kinase Activation by Blue Light

“…[9][10][11][12] As reported previously (ref. 13), the Cys → Ala mutation within LOV2 of Arabidopsis thaliana phot1 (C517A) results in a loss of light-induced receptor autophosphorylation when expressed in insect cells (Fig. 1A), demonstrating that LOV2 is the principle photosensor regulating phot1 kinase activity.…”

“…13,21 The effects observed for the Q575L mutation on phototropin kinase activity may therefore result from a reduction in the photocycle quantum yield of LOV2. However, the rate of bleaching for bacterially expressed LOV2 harboring the Gln → Leu mutation, in response to pulses of light given at different intensities, is identical to that measured for the wild-type domain (Fig.…”

“…21,22 Specifically, the side chain of a conserved glutamine residue within LOV2 (Gln 575 in Arabidopsis phot1) which forms hydrogen bonds with the FMN chromophore flips by 180° upon cysteinyl adduct formation 11,23,24 causing protein changes in the central b-sheet scaffold that forms contacts with the Ja-helix. 19,20 We have recently shown that mutation of the conserved glutamine to leucine (Q575L) attenuates light-induced autophosphorylation of Arabidopsis phot1 expressed in insect cells, 13 suggesting that this residue plays a role in transmitting the signal generated upon light-driven cysteinyl adduct formation from within LOV2 to protein changes at the LOV2 surface. An equivalent mutation when introduced into Arabidopsis phot2 (Q489L) also attenuates light-induced autophosphorylation ( Fig.…”

Higher plants use LOV to perceive blue light

“…Upon light excitation, the helix becomes unfolded and liberates the kinase domain from the inhibitory effect of LOV2 [12][13][14]. Homology modeling studies suggest that this mechanism is analogous to inhibition of PKA by the regulatory subunit PKI (Protein Kinase Inhibitor) [4].…”

Effect of computational methodology on the conformational dynamics of the protein photosensor LOV1 from Chlamydomonas reinhardtii

“…1). This subsequently mediates the activation of the kinase domain, which induces a signal in the organism via photoreceptor auto-phosphorylation [5]. A common feature of phot proteins is that the photochemical reactivity of LOV2 is required for the activation of the attached kinase domain.…”