Probing the Mammalian piRISC Cleavage Mechanism - PROJECT SUMMARY The germline contains the genetic information that will be passed to future generations. Therefore, maintaining the germline genome is essential for fertility and species survival. One mechanism of germline genome maintenance is that of the PIWI/piRNA pathway, in which small RNAs known as piRNAs interact with a PIWI Argonaute protein to form what is known as a piRNA-Induced Silencing Complex (piRISC). Through its endonuclease activity, piRISC silences repetitive elements (i.e., transposons) to protect the genome for future generations and regulates gene expression to ensure proper germ cell development and function. Loss of PIWI function leads to infertility in at least one sex in many animals, including human males. Recent work has revealed that the small zinc-finger protein, gametocyte-specific factor 1 (GTSF1), accelerates piRISC target cleavage. Loss of GTSF1 function in mice and human males leads to infertility. Preliminary kinetic evidence suggests that GTSF1 is not required for target binding or target release. However, the piRISC catalytic states associated with GTSF1 binding have not been explored. Seven GTSF1 residues have been identified as key for target cleavage, but most of these amino acids are not conserved in the other mouse GTSF paralogs, GTSF1L and GTSF2, even though they also accelerate piRISC target cleavage. This proposal seeks to test the hypothesis that mammalian GTSF proteins stabilize a catalytically active state of piRISC via key contacts with both the piRNA-target RNA duplex and PIWI protein. Aim 1 will use single-molecule FRET to probe piRISC conformational changes in the absence and presence of GTSF proteins to determine which, if any, catalytic state is stabilized in the presence of GTSF. Aim 2 will employ a high-throughput screening method to identify all amino acid positions across GTSF paralogs which are required to interact with piRISC. This study will provide insights into how GTSF1 accelerates piRISC target cleavage and determine which GTSF residues are key for its function, providing insight into why some human GTSF1 mutations lead to infertility. The proposed research will provide training in microscopy, in vitro biochemistry, and bioinformatics to prepare the fellow for a postdoc studying epigenetic inheritance and a future career as an independent investigator.
