Chug, chug, chug. An enzyme bustles down a DNA molecule, carefully matching sequences of nucleotides to the new molecule of RNA that it is synthesizing. That is possibly the simplest explanation of the flow of biological information from DNA to RNA (that I could come up with), which then codes for a protein. This universal information flow known as the Central Dogma, the fundamental tenet of biology, developed only within the past 60 years, has truly revolutionized the study of living organisms. But recently a wrinkle seems to have appeared in our understanding of the basic molecules of life and their role in the regulation of genes; a wrinkle that may bring about a second coup in our knowledge of genetics: that of microRNA.
MicroRNAs, that were discovered about 25 years ago, have remarkably been proved to be important regulators in gene expression, and they play crucial roles in almost all biological contexts including development, differentiation, inflammation, aging, and cancer. This unusual class of RNAs that are over one-thousandth the size of a typical RNA molecule, however do not code for a protein but control their expression instead. MicroRNAs (miRNAs) have continued to baffle scientists over the last two decades as increasingly fascinating and copious amounts of information are being unveiled rapidly.
miRNAs start their process as tiny, folded over hairpin structure called primary microRNAs (pri-miRNA) which is modified by the Microprocessor complex. After some further processing, mature miRNA and RNA-Induced Silencing Complex (RISC) interact with messenger RNA (mRNA) in the cytoplasm to repress translation which stops the ribosome from making protein. The Microprocessor complex is an enzyme arrangement made up of one DROSHA and two DGCR8 proteins that measures the pri-miRNA and then snips it off, resulting in precursor-microRNA (pre-miRNA).
Even though this process has been the subject of thorough investigation over the last decade, the molecular basis of the Microprocessor is still poorly understood owing to lack of knowledge of its structure. V. Narry Kim has been at the forefront of discovering and mapping out the process of miRNA biogenesis and has come out with path breaking details about the process. Last week, a team led by her, elucidated a three dimensional image of DROSHA, one part of the Microprocessor complex. Until now, no one has been able to obtain DROSHA’s crystal structure.
With this discovery, the IBS team at IBS Center for RNA Research in South Korea was able to confirm their previous findings which revealed the composition and detailed action mechanism of the Microprocessor complex. This work showed that DROSHA has two DGCR8-binding sites, giving a clear picture of how Microprocessor is assembled. After understanding its structure they were able to determine how DROSHA, along with DGCR8, interact together to determine the cleavage sites in pri-miRNA.
Attempts to purify the DROSHA protein have been hindered by technical difficulties. To maintain its structural integrity, the team co-expressed 23 amino acids from DGCR8 which binds and covers hydrophobic surface of DROSHA, keeping DROSHA intact. According to IBS researcher Jae-Sung Woo, “Without this hydrophobic interaction, the DROSHA proteins will fold abnormally and aggregate,” thus rendering them useless. Because the proteins were maintained, the team was able to perform x-ray crystallography and get the first clear image of their structure.
When they looked at DROSHA, they noticed that it bore some striking structural similarities to the Dicer enzyme, despite Dicer existing and functioning away from DROSHA that led them to hypothesize that DROSHA may have evolved from a Dicer homologue.
Understanding DROSHA’s structure is another crucial step in the process of understanding microRNA biogenesis. Achieving this imaging breakthrough opens the door for a better understanding and exciting new applications in cell reproduction. “In the future,” says researcher Sung Chul Kwon, “We are planning to solve the structure of pri-miRNA-bound Microprocessor complex.”
Source: Institute for Basic science
The original paper can be accessed here.