Background:The integrin ␣I domain undergoes a conformational change during activation. Results: The crystal structure of an activated ␣I domain is different from the reported open and closed states. Conclusion: Our structure mimics the state where the Arg 287 -Glu 317 ion pair is just broken during the activation process. Significance: The activation mechanism of the collagen receptor integrins differs from the other integrins.
The futC gene encodes a subunit of an ATP-binding cassette (ABC)-type iron transporter in Synechocystis sp. strain PCC 6803. In the present study, we have focused on the environmental regulation of futC transcription in the model organism Synechocystis sp. strain PCC 6803 and, moreover, studied the transcriptional regulation of the other transporter subunits, futA1, futA2 and futB. The steady-state amounts of the futA1, futA2, futB and futC transcripts were regulated under several conditions studied including darkness, temperature, alternative nitrogen source, salt and osmotic stresses and iron deficiency. Transcription of all subunits of the FutABC-iron transporter seems to be under similar regulation, which, according to our results, may also apply to genes encoding subunits of other transporters in Synechocystis. The sequence alignment, including sequences from six different organisms, revealed the conserved nature of FutC. Based on the sequence alignment and the structural model of FutC, the monomer consists of a nucleotide-binding domain (NBD) and a regulatory domain. The NBD is well conserved indicating completely functional ATP binding.
The potD gene encodes the bacterial substrate-binding subunit of the polyamine transport system. The uptake system, which belongs to the ABC transporters, has been characterized in Escherichia coli, but it has not been previously studied in cyanobacteria. Although the overall sequence identity between Synechocystis sp. strain PCC 6803 (hereafter Synechocystis) PotD and Escherichia coli PotD is 24%, the ligand-binding site in the constructed homology model of Synechocystis PotD is well conserved. The conservation of the five polyamine-binding residues (Asp206, Glu209, Trp267, Trp293, and Asp295 in Synechocystis PotD) between these two species indicated polyamine-binding capacity for Synechocystis PotD. The Synechocystis potD gene is functional and its expression is under environmental regulation at transcriptional as well as post-transcriptional levels. Furthermore, an in vitro binding assay with the purified recombinant PotD protein demonstrated that the Synechocystis PotD protein is able to bind polyamines and favors spermidine over putrescine. Finally, we confirmed that Synechocystis PotD plays a physiological role in the uptake of polyamines in vivo using a constructed Synechocystis potD-disruption mutant.
In higher plants, algae, and cyanobacteria, phytobilins are utilized for photosensing and/or light harvesting. Phytobilins are synthesized by ferredoxin-dependent bilin reductases (FDBRs) from biliverdin IXα (BV) derived from heme by hemeoxygenase. Phycocyanobilin:ferredoxin oxidoreductase (PcyA), one of FDBRs, sequentially reduces the vinyl group of D-ring and A-ring of BV to produce 18 1 ,18 2-dihydrobiliverdin (18EtBV) and phycocyanobilin. We reported the crystal structures of PcyA from Synechocystis sp. PCC 6803 and its complex with BV at 2.5 Å and 1.51 Å resolutions, respectively. These structures revealed that Glu76, His88 and Asp105 are located near the U-shaped BV [1] and that upon BV binding induced-fit conformational changes occur in such a way that the substrate entrance is narrowed [2]. Focusing on the structural changes in PcyA during the sequential reduction of BV, we prepared the crystals of PcyA in complex with 18EtBV and BV13, an analog of 18EtBV. These pigments were chemically synthesized. The crystal structures of PcyA-18EtBV and PcyA-BV13 were determined at 1.48 Å and 1.04 Å resolutions, respectively, revealing PcyA conformation after the reduction of D-ring vinyl group in BV. The side chain of Glu76 rotates away from D-ring to form hydrogen-bonds with both Asn62 and Tyr238. On the basis of these structures, we discuss the sequential reduction mechanism of PcyA. [1] Y.
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