부산대학교 도서관

Ribosomes are complex macromolecules assembled from 4 rRNAs and 79 ribosomal proteins (RPs). Their assembly is organized in a highly hierarchical manner, which is thought to avoid dead-end pathways, thereby enabling efficient assembly of ribosomes in the large quantities needed for healthy cellular growth. Moreover, hierarchical assembly also can help ensure that each RP is included in the mature ribosome. Nonetheless, how this hierarchy is achieved remains unknown, beyond the examples that depend on direct RP-RP interactions, which account for only a fraction of the observed dependencies. Using assembly of the small subunit head and a disease-associated mutation in the assembly factor Ltv1 as a model system, we dissect here how the hierarchy in RP binding is constructed. A combination of data from yeast genetics, mass spectrometry, DMS probing and biochemical experiments demonstrate that the LIPHAK-disease-associated Ltv1 mutation leads to global defects in head assembly, which are explained by direct binding of Ltv1 to 5 out of 15 RPs, and indirect effects that affect 4 additional RPs. These indirect effects are mediated by conformational transitions in the nascent subunit that are regulated by Ltv1. Mechanistically, Ltv1 aids the recruitment of some RPs via direct protein-protein interactions, but surprisingly also delays the recruitment of other RPs. Delayed binding of key RPs also delays the acquisition of RNA structure that is stabilized by these proteins. Finally, our data also indicate direct roles for Ltv1 in chaperoning the folding of a key rRNA structural element, the three-helix junction j34-35-38. Thus, Ltv1 plays critical roles in organizing the order of both RP binding to rRNA and rRNA folding, thereby enabling efficient 40S subunit assembly. Author summary: Ribosomes make proteins in all cells. During that process, they are responsible not just for the correct translation of the genetic code into amino acids, but also for selecting the correct mRNAs for translation. Finally, damage in mRNAs requires active translation. Changes in ribosome numbers or ribosome composition can perturb each of these functions and can lead to diseases. Thus, a better understanding of how cells ensure efficient and correct ribosome assembly is paramount. Using a mutation that is associated with a dermatological condition, LIPHAK syndrome, we describe here novel roles for the assembly factor Ltv1 in orchestrating the assembly of RPs into the head of the small ribosomal subunit. Our experiments, a combination of mass spectrometry, RNA structure probing, genetics and biochemical assays, demonstrate that Ltv1 helps recruit some RPs, while delaying the incorporation of others, thereby establishing the hierarchy of RP binding. In addition, our results also demonstrate roles for Ltv1 in modulating rRNA folding. Finally, the data establish the identity of a previously described quality control step that is bypassed by a cancer-associated mutation. [ABSTRACT FROM AUTHOR]