Significance of Knotted Structures for Function of Proteins and Nucleic Acids - September 17-21, 2014

Significance of Knotted Structures for Function of Proteins and Nucleic Acids

Friday Abstracts

Why are Phytochromes Knotted? Katrina T. Forest , Shyamosree Bhattacharya, Anna W. Baker. University of Wisconsin-Madison, Madison, USA.

Members of the phytochrome family of photoreceptors contain a deep figure-of-eight knot. We have hypothesized that signal transduction by phytochromes is efficient because of this unusual topology. Absorption of a 700 nm photon by the chromophore directs 41 kcal/mol into the protein; the knot may rigidify the photosensory core of phytochrome so that work is done to reposition the effector domain appropriately in the transition from dark to lit state, rather than permitting energy losses to random motions. To test this model, we are studying the biochemistry and structural biology of the signal transduction pathway of the bacterial phytochromes of Deinococcus radiodurans and Ramlibacter tatouinensis. Each contains a histidine kinase effector domain, and each is predicted to be a light-regulated enzyme that phosphorylates across a homodimeric interface and subsequently transfers phosphate to a response regulator protein. We have established a robust in vitro kinase assay and our preliminary results suggest R. tatouinensis bacterial phytochrome is an autokinase that interacts specifically with its bacterial phytochrome response regulator (Brr), with lower activity under red light. We have used recombineering to generate a knotless bacterial phytochrome and will test its kinase activity and the light-dependence of that activity. In addition, we have refined the X-ray crystal structure of the R. tatouinensis Brr protein. In an unusual topology, a C-terminal extension of the canonical response regulator fold wraps around its symmetry mate and returns to the originating monomer, forming an alpha helix that packs against the globular domain. Thus, to our surprise, this single domain response regulator dimerizes to form a light-independent, noncovalent catenane. The coincidence that a knotted photoreceptor is paired with a linked response regulator dimer is an intriguing phenomenon whose biological relevance has yet to be investigated.

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