Bcp1 Is the Nuclear Chaperone of Rpl23 in Saccharomyces cerevisiae

Abstract

Edited by Linda SpremulliEukaryotic ribosomes are composed of rRNAs and ribosomal proteins. Ribosomal proteins are translated in the cytoplasm and imported into the nucleus for assembly with the rRNAs. It has been shown that chaperones or karyopherins responsible for import can maintain the stability of ribosomal proteins by neutralizing unfavorable positive charges and thus facilitate their transports. Among 79 ribosomal proteins in yeast, only a few are identified with specific chaperones. Besides the classic role in maintaining protein stability, chaperones have additional roles in transport, chaperoning the assembly site, and dissociation of ribosomal proteins from karyopherins. Bcp1 has been shown to be necessary for the export of Mss4, a phosphatidylinositol 4-phosphate 5-kinase, and required for ribosome biogenesis. However, its specific function in ribosome biogenesis has not been described. Here, we show that Bcp1 dissociates Rpl23 from the karyopherins and associates with Rpl23 afterward. Loss of Bcp1 causes instability of Rpl23 and deficiency of 60S subunits. In summary, Bcp1 is a novel 60S biogenesis factor via chaperoning Rpl23 in the nucleus.The ribosome is a large macromolecular complex responsible for decoding cellular genetic information into proteins. There are two ribosomal subunits; in the eukaryotes, they are the (60S) large subunit and the small (40S) subunit, each composed of both rRNAs and ribosomal proteins. In eukaryotic cells, the assembly of ribosomes occurs in the nucleolus, a subcompartment of the nucleus devoted to ribosome biogenesis. rRNAs are transcribed by RNA polymerase I and III, and the initial assembly and processing events occur co-transcriptionally. The mRNAs of ribosomal proteins are transcribed in the nucleus by RNA polymerase II and translated in the cytoplasm. Consequently, the ribosomal proteins must be imported into the nucleus for assembly with the rRNAs. After ribosome assemblies have achieved a certain stage of maturation, export factors are loaded to direct these huge complexes to cross the nuclear membrane through the nuclear pore complexes (1-6). In the cytoplasm, ribosomes undergo additional steps in the maturation process; the trans-acting factors that are exported with nascent ribosomal subunits need to be stripped off, additional ribosomal proteins are added, and in the case of the 40S subunit, the final rRNA processing occurs (7,8). The synthesis of ribosomes requires the coordination of many non-ribosomal trans-acting factors at different stages for this pathway to be completed. About 200 trans-acting factors are involved in rRNA processing, rRNA folding, protein loading, and export. To date, more trans-acting factors have continued to be identified (7, 9, 10).BCP1 is an essential gene involved in the export of Mss4 (11), a phosphatidylinositol (PI) 2 4-phosphate 5-kinase that catalyzes the phosphorylation of PI4-phosphate and acts with the PI4-kinase, Stt4p, at the plasma membrane to generate PI4,5P 2 (12, 13). Phosphoinositols are critical small signal...

A Puzzle of Life: Crafting Ribosomal Subunits

“…In addition, a heterogeneous class of proteins, collectively referred to as dedicated chaperones, specifically protects individual r-proteins and safely guides them to their assembly site on preribosomal particles [37] (Figure 2A). So far, dedicated chaperones have been shown to associate with eight of the 79 yeast r-proteins [37,39]. Notably, the majority of these already capture their r-protein clients during translation [40,41].…”

Conformational Flexibility of Proteins Involved in Ribosome Biogenesis: Investigations via Small Angle X-ray Scattering (SAXS)

“…About 200 trans-acting factors are involved in rRNA processing, rRNA folding, protein loading, and export of the ribosomal subunits [8][9][10][11]. Bcp1 is an essential protein that functions as an escortin for Rpl23 (uL14), facilitating its dissociation from the karyopherins during reimport to the nucleus and subsequent loading into the nascent 60S subunits [12]. The function of the human Bcp1 homologue (BCCIP) is unclear and may not be equivalent to that of the yeast protein.…”

Tsr4 Is a Cytoplasmic Chaperone for the Ribosomal Protein Rps2 in Saccharomyces cerevisiae

“…Chaperones often facilitate the expression of their target RPs (10,13,14,17). To test if Tsr4 similarly enhances the expression of Rps2, we transformed a vector encoding Rps2-GFP under the control of a galactose-inducible promoter into strains harboring either a wild-type or a temperature-sensitive allele of Tsr4 (tsr4-ts).…”

“…The need for a dedicated chaperone may be bypassed by increased production of its client protein (10,(14)(15)(16)(17). We asked if the temperature sensitivity of a tsr4-ts mutant could be suppressed by an increased gene dosage of RPS2.…”