In Archaea, SRP delivers proteins to the plasma membrane. In the bacteria, SRP primarily incorporates proteins into the inner membrane. Post-translational transport SRP participates also in the sorting of proteins after their synthesis has been completed (post-translational protein sorting). In eukaryotes, tail-anchored proteins possessing a hydrophobic insertion sequence at their C-terminus are delivered to the endoplasmic reticulum (ER) by the SRP. Similarly, the SRP assists post-translationally in the import of nuclear-encoded proteins to the thylakoid membrane of chloroplasts. Structure In 2005, a nomenclature for all SRP RNAs proposed a numbering system of 12 helices. Helix sections are named with a lower case letter suffix (e.g. 5a). Insertions, or helix "branches" are given dotted numbers (e.g. 9.1 and 12.1). The SRP RNA spans a wide phylogenetic spectrum with respect to size and the number of its structural features (see the SRP RNA Secondary Structure Examples, below). The smallest functional SRP RNAs have been found in mycoplasma and related species. Escherichia coli SRP RNA (also called 4.5S RNA) is composed of 114 nucleotide residues and forms an RNA stem-loop. The gram-positive bacterium Bacillus subtilis encodes a larger 6S SRP RNA which resemble the Archaeal homologs but lacks SRP RNA helix 6. Archaeal SRP RNAs possess helices 1 to 8, lack helix 7, and are characterized by a tertiary structure which involves the apical loops of helix 3 and helix 4. The eukaryotic SRP RNAs lack helix 1 and contain a helix 7 of variable size. Some protozoan SRP RNAs have reduced helices 3 and 4. The ascomycota SRP RNAs have an altogether reduced small domain and lack helices 3 and 4. The largest SRP RNAs known to date are found in the yeasts (Saccharomycetes) which acquired helices 9 to 12 as insertions into helix 5, as well as an extended helix 7. Seed plants express numerous highly divergent SRP RNAs. Motifs Four conserved features (motifs) have been identified (shown in the Figure in dark gray): the (1) SRP54 binding motif, (2) Helix 6 GNAR tetraloop motif, (3) 5e motif, and (4) UGU(NR) motif. SRP54 binding The asymmetric loop between helical sections 8a and 8b and the adjacent base paired 8b section are a prominent property of every SRP RNA. Helical section 8b contains non-Watson-Crick base pairings which contribute to the formation of a flatted minor groove in the RNA suitable for the binding of protein SRP54 (called Ffh in the bacteria). The