A mechanism by which stress hormones inhibit the immune system, which appeared to be relatively new in evolution, may actually be hundreds of millions of years old.
A protein called the glucocorticoid receptor or GR, which responds to the stress hormone cortisol, can take on two different forms to bind DNA: one for activating gene activity, and one for repressing it. In a paper published Dec. 28 in PNAS, scientists show how evolutionary fine-tuning has obscured the origin of GRâ€™s ability to adopt different shapes.
â€œWhat this highlights is how proteins that end up evolving new functions had those capacities, because of their flexibility, at the beginning of their evolutionary history,â€ says lead author Eric Ortlund, PhD, associate professor of biochemistry at Emory University School of Medicine.
GR is part of a family of steroid receptor proteins that control cellsâ€™ responses to hormones such as estrogen, testosterone and aldosterone. Our genomes contain separate genes encoding each one. Scientists think that this family evolved by gene duplication, branch by branch, from a single ancestor present in primitive vertebrates. Read more
It arises from what scientists previously described as â€œjunk DNAâ€ or â€œthe dark matter of the genome,â€ but this gene is definitely not junk. The gene Gas5Â acts as a brake on steroid hormone receptors, making it a key player in diseases such as hormone-sensitive prostate and breast cancer.
Unlike manyÂ genes scientists are familiar with, Gas5Â does not encode a protein. It gets transcribed into RNA, like manyÂ other genes, but with Gas5Â the RNA is whatâ€™s important, not the protein. The RNA accumulates in cells subjected to stress and soaks up steroid hormone receptors, preventing them from binding DNA and turning genes on and off.
Emory researchers have obtained a detailed picture of how the Gas5 RNA interacts with steroid hormone receptors. Their findings show how the Gas5 RNA takes the place of DNA, and give hints as to how it evolved.
The results were published FridayÂ in Nature Communications.
Scientists used to think that much of the genome was â€œfly-over countryâ€: not encoding any protein and not even accessed much by the cellâ€™s gene-reading machinery. Recent studies have revealed that a large part of the genome is copied into lincRNAs (long intergenic noncoding RNAs), of which Gas5 is an example. Read more