| Ribonucleoprotein Assembly in Eukaryotes: Eukaryotic cells are host to a profuse array of small, stable RNA-protein complexes (RNPs) that carry out essential functions in, e.g., RNA processing, RNA modification, telomere maintenance, and gene regulation. The RNA and protein components of these RNPs are synthesized in separate subcellular compartments, the nucleus and cytoplasm, respectively. How and where do the individual components of small RNPs get together and assemble with one another? Does assembly follow a defined order? Is assembly cooperative, ensuring that one component will assemble only when another is also present? Except in a handful of cases, the pathways of small RNP assembly are not well delineated; still less is known about possible cooperative interactions that may regulate assembly. |
|
| Figure 1. Mammalian SRP. Circles and ellipses are the SRP proteins; solid line is SRP RNA. h5-h8 are helical regions of the RNA S domain; h5 spans the Alu and S domains. The precise locations of SRP9, 14, 19 and 54 are known. From Wild, K. et al., Curr Opin Struct Biol. 12:72-81, 2002. © Elsevier Sci. Ltd.
|
|
| As a model system to investigate RNP assembly, we are determining the in
vivo mechanism of assembly of the signal recognition particle
(SRP), a ubiquitous, phylogenetically conserved, small RNP. SRP recognizes signal sequences of secreted and membrane proteins and targets them to the cell's protein translocation apparatus. In eukaryotes, SRP conveys proteins into the lumen or the membrane of the endoplasmic reticulum (ER), where they are modified post-translationally, sorted, and dispatched to other intra- or extracellular locations. SRP occupies a crucial position in the exocytic pathway, as it mediates the first of many steps that enable cells to display surface receptors, build extracellular matrix, and secrete pheromones, antibodies, hormones, and neurotransmitters. Given its relatively modest complexity and the fact that all of its components are known, SRP is an ideal system for identifying mechanisms that coordinate and regulate RNP assembly.
In animal cells, SRP consists of a 300 nucleotide RNA and
six SRP polypeptides. SRP RNA itself consists of two sequences
elements, referred to as Alu and S; the Alu element
of SRP RNA (related to the Alu family of repetitive
sequences found in rodent and mammalian genomes) is split roughly
in half by the S sequence. Two of the SRP proteins (SRP9 and 14)
bind the Alu domain, while the other four (SRP19, 54, 68,
and 72) are associated with the S element. [see Figure 1 above].
|
