Abstract
More than 170 proteins are necessary for assembly of ribosomes in eukaryotes. However, cofactors that function with each of these proteins, substrates on which they act, and the precise functions of assembly factors--e.g., recruiting other molecules into preribosomes or triggering structural rearrangements of pre-rRNPs--remain mostly unknown. Here we investigated the recruitment of two ribosomal proteins and 5S ribosomal RNA (rRNA) into nascent ribosomes. We identified a ribonucleoprotein neighborhood in preribosomes that contains two yeast ribosome assembly factors, Rpf2 and Rrs1, two ribosomal proteins, rpL5 and rpL11, and 5S rRNA. Interactions between each of these four proteins have been confirmed by binding assays in vitro. These molecules assemble into 90S preribosomal particles containing 35S rRNA precursor (pre-rRNA). Rpf2 and Rrs1 are required for recruiting rpL5, rpL11, and 5S rRNA into preribosomes. In the absence of association of these molecules with pre-rRNPs, processing of 27SB pre-rRNA is blocked. Consequently, the abortive 66S pre-rRNPs are prematurely released from the nucleolus to the nucleoplasm, and cannot be exported to the cytoplasm.
Highlights
In eukaryotes, 79 ribosomal proteins associate with ribosomal RNA to produce 40S and 60S ribosomal subunits (Woolford and Warner 1991)
RpL11 is adjacent to 5S ribosomal RNA (rRNA) and rpL5 (Fig. 1G; Tsay et al 1994; Spahn et al 2001)
That only modestly larger amounts of 27SB pre-rRNA were recovered may reflect an incomplete block in pre-rRNA processing, together with some turnover of abortive assembly intermediates. These results reveal that each of the four proteins in the subcomplex, Rpf2, Rrs1, rpL5, and rpL11, is necessary to recruit all components of the subcomplex, including 5S rRNA, into preribosomal particles
Summary
79 ribosomal proteins associate with ribosomal RNA (rRNA) to produce 40S and 60S ribosomal subunits (Woolford and Warner 1991). Subsequent steps trigger folding, modification, and processing of prerRNAs and association of additional assembly factors and ribosomal proteins in 43S and 66S assembly intermediates These pre-rRNPs undergo further maturation in the nucleolus, nucleoplasm, and cytoplasm to form functional 40S and 60S ribosomal subunits, respectively Expression of certain unstable mutant forms of yeast 5S rRNA slows processing of 27SB pre-rRNA to mature 25S and 5.8S rRNAs and diminishes production of 60S ribosomal subunits (Van Ryk et al 1992; Dechampesme et al 1999) Taken together, these results suggest that 5S rRNA assembles together with rpL5 into preribosomes at an intermediate step in particle maturation, possibly before 27SB prerRNA undergoes further processing. Functions in ribosome assembly for most large subunit ribosomal proteins remain mostly unexplored except for rpL10, which is necessary for recycling of the export adaptor Nmd (West et al 2005)
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