Cryptochromes
Introduction
Cryptochromes are a class of flavoproteins that are sensitive to blue light. They are found in both plants and animals, and are involved in regulating circadian rhythms. In plants, cryptochromes play an essential role in various developmental processes, including seedling growth, flowering, and the entrainment of circadian rhythms. In animals, cryptochromes have been implicated in the regulation of circadian rhythms, as well as in the magnetic sense of migratory birds.
Structure and Function
Cryptochromes are composed of a photolyase-like domain and a C-terminal extension. The photolyase-like domain is responsible for light absorption, while the C-terminal extension is involved in signal transduction. Cryptochromes absorb blue light through a flavin adenine dinucleotide (FAD) chromophore, which is non-covalently bound to the protein.
In response to blue light, cryptochromes undergo a conformational change that allows them to interact with other proteins and initiate signal transduction pathways. This process is known as photoactivation. In plants, photoactivated cryptochromes can inhibit the activity of phytochromes, another class of photoreceptors, leading to changes in gene expression and plant development.
Role in Plants
In plants, cryptochromes are involved in a variety of developmental processes, including seedling growth, flowering, and the entrainment of circadian rhythms. They also play a role in phototropism, the process by which plants grow towards light.
Cryptochromes are particularly important in the regulation of plant growth in response to blue light. For example, they can inhibit hypocotyl elongation, leading to shorter, bushier plants. Cryptochromes also play a role in the regulation of flowering time. In Arabidopsis thaliana, a model plant species, cryptochromes have been shown to promote flowering in response to long days.
Role in Animals
In animals, cryptochromes are primarily involved in the regulation of circadian rhythms. They are part of the core circadian clock, a molecular oscillator that generates rhythmic changes in gene expression and cellular activity.
In mammals, cryptochromes act as inhibitors of the circadian clock. They form a complex with the proteins CLOCK and BMAL1, which are positive regulators of the clock. When activated by light, cryptochromes can inhibit the activity of CLOCK and BMAL1, leading to changes in gene expression and the entrainment of circadian rhythms.
Cryptochromes have also been implicated in the magnetic sense of migratory birds. Some birds are able to sense the Earth's magnetic field and use it for navigation. This ability is thought to be mediated by a radical pair mechanism, in which cryptochromes play a key role.