THE SRP9 14 SUBUNIT OF THE HUMAN SIGNAL RECOGNITION PARTICLE BINDS TOA VARIETY OF ALU-LIKE RNAS AND WITH HIGHER AFFINITY THAN ITS MOUSE HOMOLOG/

The heterodimeric subunit, SRP9/14, of the signal recognition particle (SRP) has previously been found to bind to scAlu and scB1 RNAs in vit ro and to exist in large excess over SRP in anthropoid cells, Here we show that human and mouse SRP9/14 bind with high affinities to other A lu-like RNAs of different evolutionary ages including the neuron-speci fic BC200 RNA, The relative dissociation constants of the different RN A-protein complexes are inversely proportional to the evolutionary dis tance between the Alu RNA species and 7SL RNA. In addition, the human SRP9/14 binds with higher affinity than mouse SRP9/14 to all RNAs anal yzed and this difference is not explained by the additional C-terminal domain present in the anthropoid SRP14. The conservation of high affi nity interactions between SRP9/14 and Alu-like RNAs strongly indicates that these Alu-like RNPs exist in vivo and that they have cellular fu nctions, The observation that human SRP9/14 binds better than its mous e counterpart to distantly related Alu RNAs, such as recently transpos ed elements, suggests that the anthropoid-specific excess of SRP9/14 m ay have a role in controlling Alu amplification rather than in compens ating a defect in SRP assembly and functions.