These considerations raise the question of whether the nucleolus may stage some sort of a “preassembly” step during the production of the translational apparatus. According to this idea, a supramolecular assembly of the translational machinery would occur in the nucleolus, perhaps transiently, through the association of 5S rRNA and SRP with nascent ribosomal subunits. Such preassembly of the translational apparatus in the nucleolus could allow for a quality control step during the synthesis and processing of the various translational components. As mentioned, it already appears that this may be the case in yeast: the nucleolar presence of 5S rRNA is required for proper processing of the large subunit rRNA (Dechampesme et al. 1999
). If a nucleolar preassembly were generally important as a checkpoint for potential functionality, other translation-related factors might also be expected to be present in the nucleolus to interact with this complex. One such potential factor is transfer RNA.
Taken together, these various observations add up rather provocatively. Not only do all four translational ribonucleoproteins arise in or visit the nucleolus, some tRNAs, perhaps even aminoacylated tRNAs, are also localized there. Although this may simply be a chance spatial coincidence, it seems more likely that there is a functional significance to this congression of translational components. As mentioned above, a plausible explanation is that the four translational ribonucleoproteins interact with one another in some sort of quality control step during synthesis, processing, and/or assembly. The four ribonucleoproteins might undergo interparticle surface interactions to probatively eliminate misshaped partners arising from errors in ribonucleoprotein assembly. Such interactions might or might not be stoichiometric with respect to the four ribonucleoproteins; topological testing could be confined to transient dimeric heterotypic particle interactions or, at the other extreme, the entire tetrapartite ribonucleoprotein translational ensemble might form, with attendant binding of tRNA and other nucleolus-associated translation factors (e.g., Jiménez-García et al. 1993
). Presumably any tRNA species in the nucleolus, including the aminoacylated form (vide supra), could probe the assembled 60S ribosomal subunit's tRNA entry site, but it is particularly interesting to note that the first (albeit preliminary) report of an aminoacylated tRNA in the nucleolus involves methioninyl tRNA (Ko, Y.G., Y.-S. Kang, E.-K. Kim, W. Seol, J.E. Kim, and S. Kim. 1999. Mol. Biol. Cell.
Although one might even expect mRNA to be involved in such a quality control step, there are few reports showing the presence of mRNA in the nucleolus (although, see Bond and Wold 1993
). However, detection of specific mRNAs in the nucleolus by in situ hybridization would be expected to be difficult, so the absence of such reports does not rule out the presence of some nucleolar mRNA. In this regard, it should be mentioned that, although considerable doubt has long existed as to whether protein synthesis occurs in isolated nuclei (Goldstein 1970
; Pederson 1976
), there does exist rather convincing evidence for amino acid incorporation into isolated nucleoli (Birnstiel and Hyde 1963
; Birnstiel and Flamm 1964
; Maggio 1966
). Whatever the level of possible cytoplasmic contamination of the initial nuclear preparations in these studies, what is now understood of the cell fractionation protocols employed would suggest that cytoplasmic contaminants of the nuclei would have been significantly reduced in the subsequent nucleolar fraction (Maggio et al. 1963a
,Maggio et al. 1963b
; Bhorjee and Pederson 1973
), which nonetheless displayed a tenfold higher rate of amino acid incorporation than nuclei (Maggio 1966
). Although the significance of these observations is still unclear, they do not allow us to rule out the (unfashionable) possibility that some peptide bond formation is catalyzed by a translation preassembly complex in the nucleolus.
A final question is whether the putative interparticle associations within this preassembly complex persist during nucleocytoplasmic transport. Does there exist the possibility of coexport of two or more of the four translational ribonucleoproteins out of the nucleolus (and the nucleus)? Most of the available evidence suggests that the large and small ribosomal subunits are typically exported as separate particles, although there have been occasional suggestions of nuclear export of intact 76S ribosomes (e.g., Khanna-Gupta and Ware 1989
). In either case, it appears that 5S rRNA typically exits minimally as part of the 60S ribosomal subunit in somatic cells. At present, nothing is known about the nucleolar exit of SRP as regards piggybacking on ribosomal particles. As we have pointed out (Jacobson and Pederson 1998b
; Politz et al. 2000
), it is conceivable that SRP is coexported with the large ribosomal subunit, since there is a known affinity of the SRP for nontranslating ribosomes (Ogg and Walter 1995
). However, coexport would not be expected to necessarily be stoichiometric with respect to SRP because SRP is typically present in cells at lower concentrations than ribosomes (Reddy and Busch 1988