Researchers have uncovered more details about the tiny defenders that ensure fertility by protecting the genomes of specialized cells called germ cells, which produce eggs and sperm. The tiny genome defenders are made from short strands of RNA -- DNA's molecular cousin -- called piRNA.
"We are interested in how piRNAs are made because these molecules are part of a fundamental pathway that animals from insects to humans use to defend their germ cell genomes. We know that piRNA is essential for fertility and is also implicated in some cancers," said the senior author.
The team has revealed that a protein called Zucchini processes piRNA from a long immature form into a shorter intermediate form. That intermediate piRNA is then matured into a functional form by another protein called Trimmer. Additionally, the RNA sequence that Zucchini recognizes as its signal to cut long immature piRNA is more complex than previously thought.
Researchers began by genetically modifying silkworm ovary cells to be unable to complete the final step of piRNA maturation by Trimmer, leaving the cells with plenty of intermediate pre-piRNA to study.
Previous reports from other groups blurred the role of the Zucchini protein, a controversy attributes to the intrinsic difficulty of studying the piRNA production pathway out of the full cellular context. In fact, both Zucchini and Trimmer sit on the surface of the "energy factories" of cells, the mitochondria, but the proteins lose their original characteristics once they are purified.
The team's innovative method of studying "crude cell pellets" where the proteins can function in a more natural situation on the mitochondrial surface revealed that indeed Zucchini does cut long immature piRNA and convert it into intermediate pre-piRNA. Moreover, the researchers discovered how Zucchini recognizes where to cut the RNA strands.
After analysis, the authors identified a novel motif in the RNA sequences where Zucchini prefers to cut an immature RNA strand. Additional experiments using crude cell pellets confirmed that altering the motif prevents normal pre-piRNA production by Zucchini.
"We were surprised to know that the simple sequence signature that was believed to be the hallmark of Zucchini's recognition is in fact not essential, but a much more complex motif is preferred by Zucchini. We imagine that the piRNA pathway is so complicated because piRNA is essential to defend the genome against diverse invader sequences and protect fertility -- cells need a flexible and robust system of defense," said the author.
https://www.nature.com/articles/s41586-020-1966-9
How germ cells defend their genome against invaders
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