Rrp5 establishes a checkpoint for 60S assembly during 40S maturation

Area of Science:

• Molecular Biology
• Cell Biology
• Biochemistry

Background:

• Ribosome biogenesis is a complex process involving the coordinated assembly of ribosomal RNAs (rRNAs) and proteins into small (40S) and large (60S) subunits.
• While 40S and 60S subunits are cotranscribed, their assembly pathways are largely distinct, requiring separate machineries.
• A critical step in 40S subunit assembly is the separation of subunits, which is delayed until late stages of 60S subunit maturation, suggesting inter-pathway communication.

Purpose of the Study:

• To elucidate the molecular mechanism by which 60S subunit assembly influences 40S subunit maturation.
• To identify the key factors mediating the crosstalk between the 40S and 60S ribosomal subunit assembly pathways.
• To understand how Rrp5 acts as a checkpoint to ensure balanced production of ribosomal subunits.

Main Methods:

• Quantitative RNA binding assays to determine Rrp5 and Noc1/Noc2 interactions with rRNA.
• RNA cleavage assays to assess the activity of Rcl1 in rRNA processing.
• Analysis of assembly factor mutants, including Noc1 HEAT-repeat domain mutants, in vivo.
• Phenotypic rescue experiments involving overexpression of wild-type and mutant Rcl1.

Main Results:

• Rrp5 acts as an early assembly factor that initially inhibits 40S subunit separation by preventing Rcl1-mediated rRNA cleavage.
• Upon transcription of 25S rRNA domain I, 60S assembly factors Noc1/Noc2 bind Rrp5 and alter its RNA binding mode, enabling pre-40S rRNA processing.
• Mutations in Noc1's HEAT-repeat domain impair subunit separation, a phenotype rescued by wild-type Rcl1, highlighting the role of Noc1-Rrp5 interaction in this process.

Conclusions:

• Rrp5 functions as a crucial checkpoint protein, linking 60S subunit assembly progression to 40S subunit maturation.
• The Noc1/Noc2-Rrp5 complex dynamically regulates rRNA processing, ensuring timely and balanced production of both ribosomal subunits.
• This crosstalk mechanism prevents the accumulation of unassembled subunits, maintaining cellular homeostasis.