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UC Riverside researchers identify key cellular organelle involved in gene silencing
RIVERSIDE, Calif.— Conducting a recent study on plants, researchers at the University of California-Riverside have shown that repression of target gene expression occurs on the endoplasmic reticulum (ER), a cellular organelle that is an interconnected network of membranes—a finding they believe will expedite scientists' understanding of the mechanism of gene silencing.
In fact, the UC-Riverside study is the first to demonstrate that the ER is where miRNA-mediated translation repression occurs, according to Dr. Xuemei Chen, lead researcher on an article published April 25 in the journal Cell, and a professor of plant cell and molecular biology at the university.
Translation inhibition is a major—but poorly understood—mode of action of microRNAs (miRNAs) in plants and animals. In particular, the subcellular location where this process takes place is unknown, explains Chen. miRNA are known to regulate target genes by two major modes of action: they either destabilize the target RNAs, leading to their degradation, or they do not impact the stability of the target RNAs, but simply prevent them from being translated into proteins—a process known as translation inhibition. The end result of translation inhibition is that the genes do not get expressed. Just how miRNAs cause translational inhibition of their target genes, however, is not well understood.
"To understand how microRNAs repress target gene expression, we first need to know where microRNAs act in the cell," Chen says. "Until now, no one knew that membranes are essential for microRNA activity."
The researchers liken the ER to "flattened sacs and branching tubules" that extend like a flat balloon throughout the cytoplasm in plant and animal cells. According to Chen, the ER has two types: rough and smooth. Rough ER, which synthesizes and packages proteins, looks bumpy; smooth ER, which acts in lipid synthesis and protein secretion, resembles tubes. The ER protein AMP1, she says, is anchored in the rough ER.
"My lab has been conducting research on AMP1 for many years," she says. "It's this protein that drew our attention to the ER. First, we realized that AMP1 is involved in miRNA-mediated translational inhibition. Then, since we already knew that AMP1 is localized in the rough ER, we shifted our focus to this organelle."
The team observed that AMP1 encodes an integral membrane protein associated with ER and ARGONAUTE1, the miRNA effector and a peripheral ER membrane protein. Large differences in polysome association of miRNA target RNAs are found between wild-type and the AMP1 mutant for membrane-bound, but not total, polysomes. This, together with AMP1-independent recruitment of miRNA target transcripts to membrane fractions, shows that miRNAs inhibit the translation of target RNAs on the ER.
The study demonstrates that translation inhibition is an important activity of plant miRNAs, reveals the subcellular location of this activity and uncovers a previously unknown function of the ER, says Chen.
"Our work shows that an integral membrane protein, AMP1, is required for the miRNA-mediated target gene repression to be successful. As AMP1 has counterparts in animals, our findings in plants could have broader implications," she says.
Next, Chen says her lab will attempt to crack the mechanism of miRNA-mediated translational inhibition. They will investigate, too, how miRNAs are recruited to the ER.
"In this study, we uncover a role of AMP1 and LAMP1 in miRNA-mediated translation inhibition, but not transcript cleavage," the researchers concluded. "Given that plant miRNAs impact many aspects of plant development, it is not surprising that AMP1 alleles were isolated in a number of genetic screens focusing on various aspects of plant development. The severe developmental defects of the AMP1 LAMP1 double mutant suggest that translation inhibition is an essential activity of plant miRNAs. However, it cannot be excluded that AMP1 and LAMP1 possess miRNA-independent functions, and these functions contribute to the pleiotropic developmental defects of the AMP1 LAMP1 mutant."