Cryptochromes are flavoproteins that are sensitive to blue light. They are structurally related to blue-light-sensitive enzymes called photolyases, which are involved in the repair of ultraviolet-induced DNA damage in all organisms, except most mammals. Unlike phytochromes, cryptochromes are also found in animals, where they have an important role in circadian clocks (see Figure 15–66). Although cryptochromes are thought to have evolved from the photolyases, they do not have a role in DNA repair.
Plants and animals are thought to have evolved multicellularity and cell communication mechanisms independently, each starting from a different unicellular eukaryote, which in turn evolved from a common unicellular eukaryotic ancestor. Not surprisingly, therefore, the mechanisms used to signal between cells in animals and in plants have both similarities and differences. Whereas animals rely heavily on GPCRs and RTKs, plants rely mainly on enzyme-coupled receptors of the receptor serine/threonine kinase type, especially ones with extracellular leucinerich repeats. Various plant hormones, or growth regulators, including ethylene and auxin, help coordinate plant development. Ethylene acts through intracellular receptors to stop the degradation of specific nuclear transcription regulators, which can then activate the transcription of ethylene-responsive genes. The receptors for some other plant hormones, including auxin, also regulate the degradation of specific transcription regulators, although the details vary. Auxin signaling is unusual in that it has its own highly regulated transport system, in which the dynamic positioning of plasma-membrane-bound auxin transporters controls the direction of auxin flow and thereby the direction of plant growth. Light has an important role in regulating plant development. These light responses are mediated by a variety of light-sensitive photoproteins, including phytochromes, which are responsive to red light, and cryptochromes and phototropin, which are sensitive to blue light.
Figure 15–73 One way in which phytochromes mediate a light response in plant cells. When activated by red light, the phytochrome, which is a dimeric protein kinase, phosphorylates itself and then moves into the nucleus, where it activates transcription regulatory proteins to stimulate the transcription of red-lightresponsive genes.
How does a cell integrate the information received from its many different cell-surface receptors to make all-or-none decisions?
Much of what we know about cell signaling comes from biochemical studies of isolated proteins in test tubes. What is the precise quantitative behavior of intracellular signaling networks in an intact cell, or in an intact animal, where countless other signals and cell components might influence signaling specificity and strength?
How do intracellular signaling circuits generate specific and dynamic signaling patterns such as oscillations and waves, and how are these patterns sensed and interpreted by the cell?
Scaffold proteins and activated receptor tyrosine kinases nucleate the assembly of large intracellular signaling complexes. What is the dynamic behavior of these complexes, and how does this behavior influence downstream signaling?
Which statements are true? explain why or why not.
15–1 All second messengers are water-soluble and diffuse freely through the cytosol.
15–2 In the regulation of molecular switches, protein kinases and guanine nucleotide exchange factors (GEFs) always turn proteins on, whereas protein phosphatases and GTPase-activating proteins (GAPs) always turn proteins off.
15–3 Most intracellular signaling pathways provide numerous opportunities for amplifying the responses to extracellular signals.
15–4 Binding of extracellular ligands to receptor tyrosine kinases (RTKs) activates the intracellular catalytic domain by propagating a conformational change across the lipid bilayer through a single transmembrane α helix.
15–5 Protein tyrosine phosphatases display exquisite specificity for their substrates, unlike most serine/threonine protein phosphatases, which have rather broad specificity.