Biomedical subjects

Sezgin Ozgür

Publications and source records attributed to Sezgin Ozgür.

4 recordsLinked to original sources

PubMed2006

Analysis of autophosphorylating kinase activities of Arabidopsis and human cryptochromes.

Cryptochromes are FAD-based blue-light photoreceptors that regulate growth and development in plants and the circadian clock in animals. Arabidopsis thaliana and humans possess two cryptochromes. Recently, it was found that Arabidopsis cryptochrome 1 (AtCry1) binds ATP and exhibits autokinase activity that is simulated by blue light. Similarly, it was reported that human cryptochrome 1 (HsCry1) exhibited autophosphorylation activity under blue light. To test the generality of light stimulated kinase function of cryptochromes, we purified AtCry1, AtCry2, HsCry1, and HsCry2 and probed them for kinase activity under a variety of conditions. We find that AtCry1, which contains near stoichiometric amounts of FAD and human HsCry1 and HsCry2 (which contain only trace amounts of FAD), has autokinase activity, but AtCry2, which also contains stoichiometric amounts of FAD, does not. Finally, we find that the kinase activity of AtCry1 is not significantly affected by light or the redox status of the flavin cofactor.

Adenosine Triphosphate↗

PubMed2006

Replication fork regression in repetitive DNAs.

Among several different types of repetitive sequences found in the human genome, this study has examined the telomeric repeat, necessary for the protection of chromosome termini, and the disease-associated triplet repeat (CTG).(CAG)n. Evidence suggests that replication of both types of repeats is problematic and that a contributing factor is the repetitive nature of the DNA itself. Here we have used electron microscopy to investigate DNA structures formed at replication forks on large model DNAs containing these repeat sequences, in an attempt to elucidate the contributory effect that these repetitive DNAs may have on their replication. Visualization of the DNA revealed that there is a high propensity for a paused replication fork to spontaneously regress when moving through repetitive DNAs, and that this results in a four-way chickenfoot intermediate that could present a significant block to replication in vivo, possibly leading to unwanted recombination events, amplifications or deletions.

DNA↗

PubMed2006

The basic domain of TRF2 directs binding to DNA junctions irrespective of the presence of TTAGGG repeats.

The replication of long tracts of telomeric repeats may require specific factors to avoid fork regression (Fouché, N., Ozgür, S., Roy, D., and Griffith, J. (2006) Nucleic Acids Res., in press). Here we show that TRF2 binds to model replication forks and four-way junctions in vitro in a structure-specific but sequence-independent manner. A synthetic peptide encompassing the TRF2 basic domain also binds to DNA four-way junctions, whereas the TRF2 truncation mutant (TRF2(DeltaB)) and a mutant basic domain peptide do not. In the absence of the basic domain, the ability of TRF2 to localize to model telomere ends and facilitate t-loop formation in vitro is diminished. We propose that TRF2 plays a key role during telomere replication in binding chickenfoot intermediates of telomere replication fork regression. Junction-specific binding would also allow TRF2 to stabilize a strand invasion structure that is thought to exist at the strand invasion site of the t-loop.

Amino Acid Sequence↗

PubMed2005

Role of structural plasticity in signal transduction by the cryptochrome blue-light photoreceptor.

Cryptochromes are blue-light photoreceptors that regulate a variety of responses such as growth and circadian rhythms in organisms ranging from bacteria to humans. Cryptochromes share a high level of sequence identity with the light-activated DNA repair enzyme photolyase. Photolyase uses energy from blue light to repair UV-induced photoproducts in DNA through cyclic electron transfer between the catalytic flavin adenine dinucleotide cofactor and the damaged DNA. Cryptochromes lack DNA repair activity, and their mechanism of signal transduction is not known. It is hypothesized that a light-dependent signaling state in cryptochromes is created as a result of an intramolecular redox reaction, resulting in conformational rearrangement and effector binding. Plant and animal cryptochromes possess 30-250 amino acid carboxy-terminal extensions beyond the photolyase-homology region that have been shown to mediate phototransduction. We analyzed the structures of C-terminal domains from an animal and a plant cryptochrome by computational, biophysical, and biochemical methods and found these domains to be intrinsically unstructured. We show that the photolyase-homology region interacts with the C-terminal domain, inducing stable tertiary structure in the C-terminal domain. Importantly, we demonstrate a light-dependent conformational change in the C-terminal domain of Arabidopsis Cry1. Collectively, these findings provide the first biochemical evidence for the proposed conformational rearrangement of cryptochromes upon light exposure.

Amino Acid Sequence↗