Cryptochrome interaction partners

Birds use the magnetic field of the Earth to navigate during their annual migratory travel. The possible mechanism to explain the biophysics of this compass sense involves electron transfers within the photoreceptive protein cryptochrome. How the information about the magnetic field is passed on from cryptochrome, however, is still unknown, since it remains to be shown which other proteins or other molecules that may interact with cryptochrome.

You can read more about the avian magnetic compass here.

The ISCA1 complex

A study [Qin et al., 2016] claimed that the sensitivity to changes in the magnetic field is enhanced by a coupling to an iron rich polymer complex which couples to multiple cryptochromes. For the iron sulphur clusters to participate in the compass sense, they either need to donate an electron to a specific tryptophane in the cryptochome or accept an electron from the flavin adenine dinucleotide (FAD) co-factor in the cryptochrome.  To validate the claim, it is needed to independently reconstruct this complex and describe its interaction with Drosophila melanogaster cryptochromes. The polymer complex consists of iron sulphur containing assembly ISCA1 protein monomers with internally bound iron sulphur clusters and simultaneously binds ten cryptochromes, shown in Fig. 1. Homology modelling and crystal packing structure of the used proteins is used to construct the large cryptochrome-ISCA1 complex,  which reveals that the iron sulphur clusters are too far away to participate in any electron transfer whatsoever.

Figure 1
Figure 1 - Molecular structure of the ISCA1-Cry complex. A: Ten different cryptochromes are shown attached to the ISCA1-polymer. The surface of the ISCA1-polymer is shown in pale blue. B: A zoom in on Cry2 featuring the FAD cofactor, the tryptophan triad, and the closest iron sulphur cluster (Fe2S2) from a nearby ISCA1 monomer. Here the contact distance between the FAD cofactor and the nearest iron sulphur cluster as well as the contact distance between the  tryptophan, (Wc), and the iron sulphur cluster is indicated.

 

The dynamic behaviour of the cryptochrome-ISCA1 complex is monitored to investigate both the time-evolution of the distance between the co-factors involved in electron transfer, and the interaction energy between cryptchrome and ISCA1, to see if the cryptochromes stick to ISCA1 and if they do so consistently along the rod. As seen in Fig. 2, the interaction energy is non-homologues along the ISCA1-rod it, revealing that the complex does likely not exist in the proposed form, and the large distance between the cofactors participating in electron factors rules out that this cryptochrome interaction has any relevance to magnetoreception. A more interesting interaction partner to cryptochrome is still sought after. 

Figure 2
Figure 2 - Interaction energy between cryptochromes and the ISCA1-polymer. A: Shown are the distributions of the interaction energy between each individual cryptochrome and the ISCA1-complex. The mean interaction energy is shown in B for each individual cryptochrome in their original simulation, and in C for each individual cryptochrome from the simulation with an adjusted configuration. A tendency for the outhermost cryptochromes to be stronger bound can be seen.

 

Can ascorbate play a role?

It was proposed in [Alpha A. Lee et al.; J. R. Soc. Interface, 11, 20131063 (2014)] that perhaps ascorbate, the ionic form of ascorbic acid (vitamin C), might be involved in magnetoreception, if this small molecule could get close enough to the surface-exposed tryptophan radical that is present in cryptochrome after photoactivation, and transfer an electron - leading to a radical pair between FAD and ascorbate instead. The central question in the ascorbate hypothesis is, therefore, whether the electron transfer from ascorbate to the tryptophan radical can happen.

Can ascorbate transfer an electron to the tryptophan radical?
Figure 3 - Can ascorbate transfer an electron to the tryptophan radical on the surface of cryptochrome?

 

Ascorbate was proposed to be involved in magnetoreception because its radical form has very small hyperfine interactions, i.e. very little coupling between the unpaired electronic spin and the spins of magnetic nuclei in ascorbate. Having small hyperfine interactions was shown to be desirable for one of the radicals in a radical pair, when paired with an FAD radical, since such a radical pair was shown to have a high sensitivity to the geomagnetic field - much more sensitive than an FAD/Tryptophan radical pair.

It was shown in [Nielsen et al.; J. R. Soc. Interface, 2017] using molecular dynamics simulations that ascorbate ions can indeed get close, and bind near the tryptophan radical as illustrated in Fig. 3. Furthermore the binding time of ascorbate was studied through a large set of additional simulations, and turned out to be about 1 ns as illustrated in Fig. 4.

Analysis of binding time of ascorbate to cryptochrome.
Figure 4 - A set of 250 molecular dynamics simulations starting with an ascorbate ion bound near the surface-exposed tryptophan in cryptochrome were performed, and it was tracked how long the ascorbate ion was staying bound in each simulation. This was repeated for two different cryptochromes (from Drosophila melanogaster, DmCry, and cryptochrome 1a from Erithacus rubecula, ErCry1a). The resulting binding time was on the order of 1 ns for both cryptochromes.

 

At the same time it was shown, however, that the expected electron transfer from ascorbate to the tryptophan radical appears to be much too slow to have any impact, and it was therefore concluded that ascorbate is unlikely to play any role in magnetoreception unless exceptionally high ascorbate concentrations are found within the cryptochrome-containing cells.

Recent Publications

Atomistic Insights into Cryptochrome Interprotein Interactions, Sarafina M. Kimø, Ida Friis, Ilia A. Solov'yov, Biophysical Journal, 115, 616-628, (2018)
Double-Cone Localization and Seasonal Expression Pattern Suggest a Role in Magnetoreception for European Robin Cryptochrome 4, Anja Günther, Angelika Einwich, Emil Sjulstok, Regina Feederle, Petra Bolte, Karl-Wilhelm Koch, Ilia A. Solov'yov, Henrik Mouritsen, Current Biology, 28, 211-223, (2018)
Ascorbic acid may not be involved in cryptochrome-based magnetoreception, Claus Nielsen, Daniel R. Kattnig, Emil Sjulstok, P. J. Hore, Ilia A. Solov'yov, Journal of the Royal Society Interface, 14, 20170657, (2017)
Computational reconstruction reveals a candidate magnetic biocompass to be likely irrelevant for magnetoreception, Ida Friis, Emil Sjulstok, Ilia A. Solov'yov, Scientific Reports, 7, 13908, (2017)