The Signal Recognition Particle (SRP) is a universally conserved and essential ribonucleoprotein complex (RNP) responsible for co-translational delivery of membrane and secretory proteins to the plasma membrane in prokaryotes and to the endoplasmic reticulum in eukaryotes. In eukaryotes, the SRP consists of six proteins (the heterodimer SRP 9/14, SRP 19, SRP 54, and the heterodimer SRP 68/72) along with a large RNA moiety, the 7S RNA.There are data suggesting non-canonical function to SRP components that have not been fully characterized. SRP72 has been identified as the only SRP S domain component to possess a phosphorylation site and be involved in cleavage signalling during apoptosis. SRP68 and SRP72 were identified to interact with the tail of histone H4 and further characterise to regulate gene expression indicating their role and interaction with other cellular components outside the protein synthesis cycle.
Mass-spectrometry-based proteomic analysis of mammalian cells revealed that the heterodimer SRP68/72 interacts with a variety of proteins involved in complex formation with nucleic acid or other proteins. In particular, SRP68/72 interact with PRMT1 and PRMT5, previously identified as modulators of SRP68/72 sub-cellular localisation. Nuclear extracts from mammalian cells were used for pull down experiments and proteomic analysis revealed that recombinant human SRP54 interacts through RNA-binding with a variety of splicing factors, ribonucleoprotein complexes associated with nascent transcripts and paraspeckle components.
In different mammalian cells SRP54, previously reported to be only present in the cytoplasm, localises mainly to the cytoplasm with some nuclear expression; in Hela synchronised cells, the nuclear staining seems to be enriched during G2-M phase of the cell cycle. Upon entry into mitosis SRP54 is excluded from condensated chromatin, becoming diffusely distributed through the cytosol. SRP19, SRP68 and 72 show a diffuse cytoplasmic and nuclear staining overall the cell cycle.