CHARACTERIZATION OF A UNIQUE HIGH MOBILITY GROUP (HMG) BOX DOMAIN OF MOUSE MAELSTROM

Maelstrom (MAEL) is a gonad-specific protein that is associated with the piRNA pathway and is involved in silencing of transposable elements (TEs). In the absence of MAEL, the function of the piRNA pathway is perturbed and retrotransposons become expressed, causing infertility. Here, I utilize sequence alignments, tertiary structural analysis and biochemical approaches to characterize MAEL’s N-terminal high-mobility group (HMG)-box domain, which is required for function in Drosophila. I have compared mouse and fruit fly MAEL HMG-box with sequence specific sex-determining region Y (SRY) HMG-box, and non-sequence specific high mobility group protein B1 (HMGB1) HMG-box-A. Sequence and structural comparisons revealed novel arrangements of residues and tertiary structural regions (“propeller,” “hook”) that distinguish MAEL HMG-box from previously described HMG-boxes. These characteristics are highly conserved within vertebrate domains but diverged in invertebrate domains. Gel-shift assays show that MAEL HMG-box does not bind to B-type helical or cytosine methylation modified double-stranded (ds) DNA, but strongly binds to structured DNA four-way junctions. More importantly, MAEL HMG-box binds to RNA. It binds to dsRNA, hairpins, and 4WJs forming stronger complexes with each substrate consecutively. Binding to junctions depends on the conserved arginine residues within the “hook” and “propeller” regions. MAEL HMG-box also binds to large RNA fragments from sequence regions enriched in the MAEL immunoprecipitates and not to RNA that was not enriched. These results indicate that MAEL HMG-box is an RNA-binding domain with preference for large, structured substrates. Accordingly, MAEL HMG-box may bestow the RNA-binding capabilities on MAEL protein, allowing for proper selection of target-RNA molecules and their delivery to the piRNA pathway