Alu RNA pseudoknot alterations influence SRP9/SRP14 association

There are over 1 million Alu elements in the human genome which can be transcribed into discrete, RNA polymerase III transcribed noncoding Alu RNAs. These Alu RNAs often interact with and are regulated by the protein heterodimer SRP9/SRP14. This interaction is dependent on a 5' pseudoknot domain in the Alu RNA that is thought to be held together by a canonical nucleotide triad within a U-turn motif. Herein, we discover a significant reduction in BC200 expression after mutation of a critical guanosine in the U-turn motif within its pseudoknot domain. We studied a recently discovered short human Alu RNA, EB120, which lacks the canonical Alu RNA U-turn nucleotide triad. We tested the expression of EB120 in 18 different human cell lines and tissues. EB120 was found to lack association with SRP9/SRP14 in a cellular context. Small-angle X-ray scattering followed by atomistic computation structure prediction suggests the BC200 Alu domain and its U-turn mutant both possess a canonical Alu RNA fold, while EB120 lacks one. Our results highlight the structural diversity of Alu RNA, and the impact mutations may have on Alu RNA function.