THE HETERODIMERIC SUBUNIT SRP9 14 OF THE SIGNAL RECOGNITION PARTICLE FUNCTIONS AS PERMUTED SINGLE POLYPEPTIDE-CHAIN/

The targeting of nascent polypeptide chains to the endoplasmic reticul um is mediated by a cytoplasmic ribonucleoprotein, the signal recognit ion particle (SRP). The 9 kD (SRP9) and the 14 kD (SRP14) subunits of SRP are required to confer elongation arrest activity to the particle. SRP9 and SRP14 form a heterodimer which specifically binds to SRP RNA . We have constructed cDNAs that encode single polypeptide chains comp rising SRP9 and SRP14 sequences in the two possible permutations linke d by a 17 amino acid peptide. We found that both fusion proteins speci fically bound to SRP RNA as monomeric molecules folded into a heterodi mer-like structure. Our results corroborate the previous hypothesis th at the authentic heterodimer binds to SRP RNA in equimolar ratio. In a ddition, both fusion proteins conferred elongation arrest activity to SRP(-9/14), which lacks this function, and one fusion protein could fu nctionally replace the heterodimer in the translocation assay. Thus, t he normal N-and C-termini of both proteins have no essential role in f olding, RNA-binding and in mediating the biological activities. The po ssibility to express the heterodimeric complex as a single polypeptide chain facilitates the analysis of its functions and its structure in vivo and in vitro.