[show abstract][hide abstract] ABSTRACT: Chloroplast photorelocation movement towards weak light and away from strong light is essential for plants to adapt to the fluctuation of ambient light conditions. In the previous study, we showed that blue light receptor phototropins mediated blue light-induced chloroplast movement in Arabidopsis by regulating short actin filaments localized at the chloroplast periphery (cp-actin filaments) rather than actin cables in the cytoplasm. However, the signaling pathway for the chloroplast photorelocation movement is still unclear. We also identified JAC1 (J-domain protein required for chloroplast accumulation response 1) as an essential component for the accumulation response and dark positioning in Arabidopsis. We recently determined the crystal structure of the J-domain of JAC1. The JAC1 J-domain has a positively charged surface, which forms a putative interface with the Hsc70 chaperone by analogy to that of bovine auxilin. Furthermore, the mutation of the highly conserved HPD motif in the JAC1 J-domain impaired the in vivo activity of JAC1. These data suggest that JAC1 cochaperone activity with HSC70 is essential for chloroplast photorelocation movement.
[show abstract][hide abstract] ABSTRACT: An auxilin-like J-domain-containing protein, JAC1, is necessary for chloroplast movement in Arabidopsis thaliana, to capture photosynthetic light efficiently under weak light conditions. Here, we performed crystallographic and functional analyses of the J-domain of JAC1. The crystal structure of the J-domain is quite similar to that of bovine auxilin, and possesses a similar positively charged surface, which probably forms the interface with the Hsp70 chaperone. The mutation of the highly conserved HPD motif of the JAC1 J-domain abrogated the chloroplast photorelocation response. These results suggest that the requirement of JAC1 in chloroplast photorelocation movement is attributable to the J-domain's cochaperone activity.
Plant and Cell Physiology 08/2010; 51(8):1372-6. · 4.13 Impact Factor
[show abstract][hide abstract] ABSTRACT: Herbicides targeting photosystem II (PSII) block the electron transfer beyond Q(A) by binding to the Q(B) site. Upon binding, the redox potential of Q(A) shifts differently depending on the types of herbicides. In this study, we have investigated the structures, interactions, and locations of phenolic herbicides in the Q(B) site to clarify the molecular mechanism of the Q(A) potential shifts by herbicides. Fourier transform infrared (FTIR) difference spectra upon photoreduction of the preoxidized non-heme iron (Fe(2+)/Fe(3+) difference) were measured with PSII membranes in the presence of bromoxynil or ioxynil. The CN and CO stretching vibrations of these phenolic herbicides were identified at 2215-2200 and 1516-1505 cm(-1), respectively, in the Fe(2+)/Fe(3+) difference spectra. Comparison with the spectra of bromoxynil in ethanol solutions along with density functional theory analysis strongly suggests that the phenolic herbicides take a deprotonated form in the binding pocket. In addition, the CN stretching, NH bending, and NH stretching vibrations of a His side chain, which were found at 1109-1101, 1187-1185, and 3000-2500 cm(-1), respectively, in the Fe(2+)/Fe(3+) difference spectra, showed characteristic features in the presence of phenolic herbicides. These signals are probably attributed to D1-His215, one of the ligands to the non-heme iron. Docking calculations for herbicides to the Q(B) pocket confirmed the binding of deprotonated bromoxynil to D1-His215 at the CO group, whereas the protonated form of bromoxynil and DCMU were found to bind to the opposite side of the pocket without an interaction with D1-His215. From these results, it is proposed that a strong hydrogen bond of the phenolate CO group with D1-His215 induces the change in the hydrogen bond strength of the Q(A) CO group through the Q(A)-His-Fe-His-phenolate bridge causing the downshift of the Q(A) redox potential.
[show abstract][hide abstract] ABSTRACT: The redox potential of Q(A) in photosystem II (PSII) is known to be lower by approximately 100 mV in the presence of phenolic herbicides compared with the presence of DCMU-type herbicides. In this study, the structural basis underlying the herbicide effects on the Q(A) redox potential was studied using Fourier transform infrared (FTIR) spectroscopy. Light-induced Q(A)(-)/Q(A) FTIR difference spectra of Mn-depleted PSII membranes in the presence of DCMU, atrazine, terbutryn, and bromacil showed a strong CO stretching peak of Q(A)(-) at 1,479 cm(-1), while binding of phenolic herbicides, bromoxynil and ioxynil, induced a small but clear downshift by approximately 1 cm(-1). The CO peak positions and the small frequency difference were reproduced in the S(2)Q(A)(-)/S(1)Q(A) spectra of oxygen-evolving PSII membranes with DCMU and bromoxynil. The relationship of the CO frequency with herbicide species correlated well with that of the peak temperatures of thermoluminescence due to S(2)Q(A)(-) recombination. Density functional theory calculations of model hydrogen-bonded complexes of plastoquinone radical anion showed that the small shift of the CO frequency is consistent with a change in the hydrogen-bond structure most likely as a change in its strength. The Q(A)(-)/Q(A) spectra in the presence of bromoxynil, and ioxynil, which bear a nitrile group in the phenolic ring, also showed CN stretching bands around 2,210 cm(-1). Comparison with the CN frequencies of bromoxynil in solutions suggested that the phenolic herbicides take a phenotate anion form in the Q(B) pocket. It was proposed that interaction of the phenolic C-O(-) with D1-His215 changes the strength of the hydrogen bond between the CO of Q(A) with D2-His214 via the iron-histidine bridge, causing the decrease in the Q(A) redox potential.
Photosynthesis Research 05/2008; 98(1-3):159-67. · 3.15 Impact Factor
[show abstract][hide abstract] ABSTRACT: Previous evidence suggested that transfer RNAs (tRNAs) cross the nuclear envelope to the cytosol only once after maturing in the nucleus. We now present evidence for nuclear import of tRNAs in yeast. Several export mutants accumulate mature tRNAs in the nucleus even in the absence of transcription. Import requires energy but not the Ran cycle. These results indicate that tRNAs shuttle between the nucleus and cytosol.