The SRP9/14 subunit of the signal recognition particle (SRP) is present in more than 20-fold excess over SRP in primate cells and exists primarily free but also in complex with small cytoplasmic Alu RNAs.

Abstract

The heterodimeric protein SRP9/14 bound to the Alu sequences of SRP RNA is essential for the translational control function of the signal recognition particle (SRP). The Alu RNAs of primate cells are believed to be derived from SRP RNA and have been shown to bind to an SRP14-related protein in vitro. We have used antibodies to characterize SRP9/14 and examine its association with small RNAs in vivo. Although SRP9 proteins are the same size in both rodent and primate cells, SRP14 subunits are generally larger in primate cells. An additional alanine-rich domain at the C-terminus accounts for the larger size of one human isoform. Although the other four SRP proteins are largely assembled into SRP in both rodent and primate cells, we found that the heterodimer SRP9/14 is present in 20-fold excess over SRP in primate cells. An increased synthesis rate of both proteins may contribute to their accumulation. The majority of the excess SRP9/14 is cytoplasmic and does not appear to be bound to any small RNAs; however, a significant fraction of a small cytoplasmic Alu RNA is complexed with SRP9/14 in a 8.5 S particle. Our findings that there is a large excess of SRP9/14 in primate cells and that Alu RNAs are bound to SRP9/14 in vivo suggest that this heterodimeric protein may play additional roles in the translational control of gene expression and/or Alu transcript metabolism.

Useful ‘junk’: Alu RNAs in the human transcriptome

“…1a). Each arm is related to the Alu portion of SRP RNA in terms of sequence and secondary structure and can bind the cognate SRP protein SRP9/14 in vitro [11] and in vivo [12,13]. Recent results from our group revealed that synthetic Alu RNPs (ribonucleoproteins), composed of Alu RNA in complex with SRP9/14, and naked Alu RNA have opposite effects on protein synthesis in cell-free translation systems.…”

“…Further investigations of the mechanism by which they influence protein translation showed that both of them act at the level of translation initiation [14]. A large fraction of the protein SRP9/14 is not assembled into SRP in primate cells [12]. The free SRP9/14 may bind Alu RNA and the occurrence of Alu RNP in vivo seems therefore very likely.…”

“…The free SRP9/14 may bind Alu RNA and the occurrence of Alu RNP in vivo seems therefore very likely. In addition, Alu RNA migrates as a high molecular weight complex in density gradients of HeLa cell extracts [12,15], and Alu RNAs expressed in response to adenovirus infection are assembled in SRP9/14-containing RNPs [13]. Expression of free Alu RNA in vivo was shown to stimulate translation of co-transfected reporter genes [16,17].…”

“…One piece of missing data is then the regulation of SRP9/14 expression. The excess of SRP9/14 over SRP was only observed in immortalized cell lines [12], and the expression levels of SRP9/14, relative to SRP, will have to be investigated in different tissues and under different physiological conditions to better characterize the potential regulation of Alu RNA activity. Moreover, as most of the previous studies concerning Alu RNA transcription were performed using cultured cancer cells [8,9,15,18,19], it would also be interesting to measure the level of Alu RNA in different tissues or in cultured primary cells.…”

Crystallization and preliminary crystallographic analysis of the signal recognition particle SRPΦ14‐9 fusion protein

“…It is possible that SRP9/14 might possess a new RNA binding motif which may be generated upon heterodimerization since neither protein alone binds specifically to SRP RNA. Recently, it has been shown that human SRP9/14 in vivo [7], and SRP14 in vitro [25] can associate as complexes with scAlu RNA in so-called Alu particles. Furthermore, the human SRP14 is larger than its murine counterpart, this difference being due to a C-terminal alanine/threonine-rich extension [25].…”