HUMAN SIGNAL RECOGNITION PARTICLE (SRP) ALU-ASSOCIATED PROTEIN ALSO BINDS ALU INTERSPERSED REPEAT SEQUENCE RNAS - CHARACTERIZATION OF HUMANSRP9

Nearly 1 million interspersed Alu elements reside in the human genome. Alu retrotransposition is presumably mediated by full-length Alu tran scripts synthesized by RNA polymerase III, while some polymerase III-s ynthesized Alu transcripts undergo 3'-processing and accumulate as sma ll cytoplasmic (sc) RNAs of unknown function. Interspersed Alu sequenc es also reside in the untranslated regions of some mRNAs. The Alu sequ ence is related to a portion of the 7SL RNA component of signal recogn ition particle (SRP), This region of 7SL RNA together with 9- and 14-k Da polypeptides (SRP9/14) regulates translational elongation of riboso mes engaged by SRP. Here we characterize human (h) SRP9 and show that it, together with hSRP14 (SRP9/14), forms the activity previously iden tified as Alu RNA-binding protein (REP), The primate-specific C-termin al tail of hSRP14 does not appreciably affect binding to scAlu RNA. K- d values for three Alu-homologous scRNAs were determined using Alu REP (SRP9/14) purified from HeLa cells. The Alu region of 7SL, scAlu, and scB1 RNAs exhibited K-d values of 203 pM, 318 pM, and 1.8 nM, respect ively, Finally, Alu REP can bind with high affinity to synthetic mRNAs that contain interspersed Alus in their untranslated regions.