Synchronizing Nuclear Import of Ribosomal Proteins with Ribosome Assembly

Abstract

Ribosomal proteins are synthesized in the cytoplasm, before nuclear import and assembly with ribosomal RNA (rRNA). Little is known about coordination of nucleocytoplasmic transport with ribosome assembly. Here, we identify a transport adaptor, symportin 1 (Syo1), that facilitates synchronized coimport of the two 5S-rRNA binding proteins Rpl5 and Rpl11. In vitro studies revealed that Syo1 concomitantly binds Rpl5-Rpl11 and furthermore recruits the import receptor Kap104. The Syo1-Rpl5-Rpl11 import complex is released from Kap104 by RanGTP and can be directly transferred onto the 5S rRNA. Syo1 can shuttle back to the cytoplasm by interaction with phenylalanine-glycine nucleoporins. X-ray crystallography uncovered how the a-solenoid symportin accommodates the Rpl5 amino terminus, normally bound to 5S rRNA, in an extended groove. Symportin-mediated coimport of Rpl5-Rpl11 could ensure coordinated and stoichiometric incorporation of these proteins into pre-60S ribosomes.R ibosomes perform their role in translation in the cytoplasm, but ribosome assembly occurs predominantly in a specialized nuclear compartment, the nucleolus (1-4). The construction of ribosomes follows an ordered assembly of~80 ribosomal proteins (r-proteins) and four ribosomal RNAs (rRNAs) into a small (40S) and large (60S) ribosomal subunit. This process is spatially and temporally coordinated, starting with cotranscriptional assembly of a first preribosomal particle (90S) in the nucleolus that is subsequently separated into pre-40S and pre-60S ribosomes, which follow independent processing and maturation steps before export into the cytoplasm (5-8). R-proteins are synthesized in the cytoplasm and are imported into the nucleus by nuclear import receptors of the importinb/karyopherin family (9, 10). These transport receptors recognize different types of nuclear localization sequences (NLSs), and hence a number of import receptors have been implicated in decoding NLSs of r-proteins in a redundant way (9, 11). After nuclear import and before incorporation into nascent ribosomes, r-proteins are released from the transport receptor by its interaction with RanGTP (12). At present, it is thought that each r-protein is individually transported into the nucleus by its import receptor. However, a number of r-proteins form functional clusters on the ribosomal surface or assemble at distinct temporal or spatial entry points during ribosome formation (13-15), thus raising the possibility of coordinated nuclear import and assembly of r-proteins.One such pair of functionally related r-proteins is Rpl5 and Rpl11, which are close to each other on the mature 60S subunit and bind to opposite sites on the 5S rRNA ( Fig.

A Puzzle of Life: Crafting Ribosomal Subunits

“…A specific feature of LSU assembly is the incorporation of a preformed 5S RNP ( Figure 2J) comprising the 5S RNA and the rproteins uL5 and uL18. Formation of the 5S RNP is facilitated by the transport adaptor Syo1, which couples nuclear import of uL5 and uL18 with 5S rRNA association [90,91]. The subsequent incorporation of the 5S RNP module involves the Rpf2-Rrs1 heterodimer and has already occurred within the Nsa1-defined particle [25,86,[92][93][94] (Figure 2K).…”

Bcp1 Is the Nuclear Chaperone of Rpl23 in Saccharomyces cerevisiae

“…It has been shown that karyopherins or chaperones interact with ribosomal proteins to maintain their solubility and also facilitate their loading into nascent ribosomal subunits (33)(34)(35)(36)(37)(38)(39)(40)(41)(42).…”

“…Rrb1 has direct interaction with Rpl3 (uL3), and overexpression of Rrb1 increased the level and nuclear accumulation of Rpl3 (36). Syo1 facilitates coimport of Rpl5 (uL18) and Rpl11 (uL5) and chaperones 5S RNP assembly (33,38). Acl4 is a chaperone of Rpl4 (uL4) and accompanies Rpl4 from the cytoplasm to the loading onto the pre-60S in the nucleus (39,41).…”

60S ribosome biogenesis requires rotation of the 5S ribonucleoprotein particle

“…In contrast, 5S rRNA is transcribed separately by RNA polymerase III, and its precursor matures to the 5S rRNA independently from the pre-35S RNA processing pathway 9,10 . Subsequently, 5S rRNA recruits two r-proteins, L5 and L18 (previously called Rpl11 and Rpl5 in yeast, respectively), to form the 5S ribonucleoprotein (RNP) particle, which becomes incorporated en bloc into the evolving pre-60S subunit [11][12][13] . In the mature 80S ribosome, the 5S rRNA is located within the central protuberance of the 60S subunit, where it is involved in intersubunit interactions, mediated by a number of protein-RNA and RNA-RNA contacts 14,15 .…”