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Telomere-binding proteins have recently been recognised not only as necessary building blocks of telomere structure, but namely as components which are of central importance to telomere metabolism being involved in regulation of telomere length as well as in protective (capping) function of telomeres. Although the knowledge on plant telomeric DNA-binding proteins lags behind that in human and yeast, recent data show both analogies and plant-specific features in the composition and interactions of telomeric proteins. This review focuses primarily on proteins with known amino acid sequence. These can be classified into following groups: 1) the family of proteins with Myb domain at C-terminus, 2) proteins with Myb domain at N-terminus, both binding double-stranded DNA of telomeric repeats TTTAGGG, 3) the single-stranded DNA-binding proteins, and 4) other proteins that act also in non-telomeric chromatin regions. Proteins with C-terminal Myb domain reported as IBP family were previously found in human, whereas Smh family representing proteins with Myb domain at N-terminus was identified only in plants. Also RRM family of the single-stranded DNA-binding proteins is likely to be plant specific.
Telomere-binding proteins have recently been recognised not only as necessary building blocks of telomere structure, but namely as components which are of central importance to telomere metabolism being involved in regulation of telomere length as well as in protective (capping) function of telomeres. Although the knowledge on plant telomeric DNA-binding proteins lags behind that in human and yeast, recent data show both analogies and plant-specific features in the composition and interactions of telomeric proteins. This review focuses primarily on proteins with known amino acid sequence. These can be classified into following groups: 1) the family of proteins with Myb domain at C-terminus, 2) proteins with Myb domain at N-terminus, both binding double-stranded DNA of telomeric repeats TTTAGGG, 3) the single-stranded DNA-binding proteins, and 4) other proteins that act also in non-telomeric chromatin regions. Proteins with C-terminal Myb domain reported as IBP family were previously found in human, whereas Smh family representing proteins with Myb domain at N-terminus was identified only in plants. Also RRM family of the single-stranded DNA-binding proteins is likely to be plant specific.
Adenylyl cyclase (AC) is an enzyme that catalyses the formation of the second messenger molecule, 3′,5′-cyclic adenosine monophosphate (cAMP) from 5′-adenosine triphosphate (ATP). cAMP, in turn, regulates key physiological processes such as cell division, growth, reproduction, development and response to stress. However, while cAMP is increasingly becoming an important signalling molecule in higher plants, the identification of plant ACs has somewhat remained so slow. In Arabidopsis thaliana alone, only twelve ACs have so far been identified, yet considering the number and diverse nature of processes known to be cAMP-dependent in this plant, these identified ACs are still very much few to account for that. Notably, an additional protein in this plant, termed linker histone-like (AtLHL) protein (encoded by the At3g18035 gene), is annotated to be an AC as result of it containing a putative centre identical to the one commonly found in the other twelve previously confirmed Arabidopsis ACs. In addition, AtLHL is mostly involved in a number of key cellular processes such as heterochromatin formation, DNA repair, apoptosis, embryogenesis, reproduction and disease resistance that are all modulated by cAMP, yet AtLHL still remains unconfirmed as an AC. As a result, we targeted this protein in this study to determine if it is indeed an AC. To begin with, we used computational analysis to assess the 3-dimensional (3D) structure of AtLHL and found that its AC centre is solvent-exposed, amenable to the unhindered access of ATP as a substrate for catalysis. Next, we cloned, partially expressed and affinity purified a truncated version of this protein (AtLHL301−480), followed by assessment of its probable AC activity. Through enzyme immunoassay and mass spectrometry, we showed that the recombinant AtLHL301−480 protein can generate cAMP from ATP in vitro in a manganese-dependent manner that is enhanced by calcium and hydrogen carbonate. In addition, we also showed that the recombinant AtLHL301−480 protein can complement AC-deficiency (cyaA mutation) in SP850 cells when expressed in this mutant Escherichia coli host strain. We then used electrochemistry to evaluate the molecular interaction of AtLHL301−480 with its co-factors and modulators during catalysis and activation, respectively, and found that the protein does this physically. This observation then prompted us to specifically search for the presence (and possibly frequency) of calcium-binding sites within the AtLHL protein. Through in silico analysis and bioinformatic studies, a single binding site in form of a 16-residue calmodulin-binding sequence was predicted. Lastly, we then evaluated the reaction kinetics of AtLHL301−480 and determined that the protein has a Km constant of 0.7 mM and a Vmax constant of 9.2 fmol/min/μg protein. All in all, our study provided adequate evidence in a multi-faceted manner that LHL from A. thaliana is a bona fide AC, whose activity might be involved in control and molecular regulation of the various functions of this protein in this plant.
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