only mature mRNA molecules that have completed the processes of capping, polyadenylation, splicing,
editing, and export to the cytoplasm.
7.4 RNAs Involved in Export and Transport
7.4.1 Transport of RNA
In eukaryotes, RNAs are typically synthesized and processed in one place, usually the nucleus, and then
transported to another site, usually the cytoplasm, for function. There are specific sequences or structures
in most target RNA molecules that bind transport proteins and help direct an RNA molecule to its func-
tional destination.^59 Like so many other regulatory signals, these targeting structures are located commonly
in the 3 -untranslated region (3-UTR)of transported RNAs (Figure 7.16).
The 3-UTR structures that are important for RNA trafficking are highly diverse. Some 3-UTR targeting
sequences are short, such as the 21-nucleotide sequence that binds protein hnRNP A2, which targets tran-
scripts such as the mRNA that codes for a myelin protein. Other 3-UTR sequences adopt more complex
structures, such as those involved in RNA localization during development (e.g., grk mRNA in Drosophila),
or those that bind the ZIP-codefamily of targeting proteins (e.g., -actin or Vg1 mRNAs). In almost all
cases, the RNA binding proteins that bind these 3-UTR structures are involved in other processes, such that
pre-mRNA splicing, capping, and other events are interdependent and linked to transport of mRNA.
7.4.2 RNA that Transports Protein: the Signal Recognition Particle
RNA structures are not only involved in the shuttling of RNA molecules, they are also essential for the
transport of proteins. This is exemplified by the role of 7SL RNAin the signal recognition particle
(SRP).^60 The SRP is a RNP complex that is present in all three kingdoms of life (Figure 7.32). Although
its complexity has increased with evolution, many of the major components in SRP (such as a conserved
280 Chapter 7
Figure 7.32 SRP complexes from the three kingdoms of life. (a) The SRP becomes increasingly complex from eubac-
teria to archaea, to mammals (left to right). All of them have an “SRP-54”-like component (yellow
ellipse) and an “S” domain. Additional components (such as the Alu domain and SRP 9) have been
added in more complex organisms
(Reprinted from Ref. 60. © (2003), with permission from Elsevier)
(b) Crystal structure of SRP “S” domain RNA in complex with signal peptide and associated proteins
(Reprinted from Ref. 77. © (2002), with permission from Macmillan Publications Ltd)