Cryptochrome magnetoreception: four tryptophans could be better than three
The biophysical mechanism of the magnetic compass sensor in migratory songbirds is thought to involve photo-induced radical pairs formed in cryptochrome (Cry) flavoproteins located in photoreceptor cells in the eyes. In Cry4a – the most likely of the six known avian cryptochromes to have a magnetic...
| Main Authors: | , , , , |
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| Format: | Journal article |
| Language: | English |
| Published: |
Royal Society
2021
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| Summary: | The biophysical mechanism of the magnetic compass sensor in migratory songbirds is thought to
involve photo-induced radical pairs formed in cryptochrome (Cry) flavoproteins located in
photoreceptor cells in the eyes. In Cry4a – the most likely of the six known avian cryptochromes to
have a magnetic sensing function – four radical pair states are formed sequentially by the stepwise
transfer of an electron along a chain of four tryptophan residues to the photo-excited flavin. In purified
Cry4a from the migratory European robin, the third of these flavin-tryptophan radical pairs is more
magnetically sensitive than the fourth, consistent with the smaller separation of the radicals in the
former. Here, we explore the idea that these two radical pair states of Cry4a could exist in rapid dynamic
equilibrium such that the key magnetic and kinetic properties are weighted averages. Spin dynamics
simulations suggest that the third radical pair is largely responsible for magnetic sensing while the fourth
may be better placed to initiate magnetic signalling particularly if the terminal tryptophan radical can
be reduced by a nearby tyrosine. Such an arrangement could have allowed independent optimisation of
the essential sensing and signalling functions of the protein. It might also rationalise why avian Cry4a
has four tryptophans while cryptochromes from plants have only three. |
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