HUMAN SIGNAL RECOGNITION PARTICLE (SRP) ALU-ASSOCIATED PROTEIN ALSO BINDS ALU INTERSPERSED REPEAT SEQUENCE RNAS - CHARACTERIZATION OF HUMAN SRP9

Nearly 1 million interspersed Alu elements reside in the human genome. Alu retrotransposition is presumably mediated by full-length Alu transcripts synthesized by RNA polymerase III, while some polymerase III-synthesized Alu transcripts undergo 3'-processing and accumulate as small cytoplasmic (sc) RNAs of unknown function. Interspersed Alu sequences also reside in the untranslated regions of some mRNAs. The Alu sequence is related to a portion of the 7SL RNA component of signal recognition particle (SRP), This region of 7SL RNA together with 9- and 14-kDa polypeptides (SRP9/14) regulates translational elongation of ribosomes engaged by SRP. Here we characterize human (h) SRP9 and show that it, together with hSRP14 (SRP9/14), forms the activity previously identified as Alu RNA-binding protein (REP), The primate-specific C-terminal 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, respectively, Finally, Alu REP can bind with high affinity to synthetic mRNAs that contain interspersed Alus in their untranslated regions.