90S preribosome assembly

Definition

Target type: biologicalprocess

The aggregation, arrangement and bonding together of proteins and RNA molecules to form a 90S preribosome. The 90S preribosome represents the complex that forms on the primary rRNA transcript before it splits into the small subunit and large subunit portions. [GOC:krc, GOC:mah, GOC:tb]

The 90S preribosome is the initial complex in eukaryotes that will eventually become the mature 80S ribosome, the protein synthesis machinery. Its assembly is a complex process involving numerous proteins, RNAs, and energy, and it can be broadly divided into several steps:

**1. Formation of the Small Subunit (40S)**
- The 18S rRNA is transcribed in the nucleolus and associates with a number of ribosomal proteins (including uS3, uS5, uS7, uS10, uS11, uS14, uS15, uS16, uS17, uS18, uS19, uS20, uS24, uS25, uS27, uS28, uS29, uS30, uS31) to form the 40S subunit.
- This process starts with the formation of small ribosomal subunit precursors (90S pre-rRNA) and involves the assembly of various proteins.
- The 40S subunit matures through a series of conformational changes, facilitated by the binding and release of various proteins.

**2. Formation of the Large Subunit (60S)**
- The 5.8S, 5S, and 25S rRNAs are transcribed in the nucleolus and associate with ribosomal proteins (including uL3, uL4, uL5, uL11, uL13, uL14, uL16, uL18, uL23, uL24, uL25, uL26, uL29, uL30, uL32, uL33, uL34, uL35, uL36, uL37, uL39, uL40, uL41, uL43, uL44, uL45, uL46, uL48, uL50, uL51, uL52, uL54, uL55, uL56, uL58, uL59, uL60, uL61, uL63, uL64, uL65, uL66, uL67, uL68, uL69, uL70, uL71, uL72, uL73, uL74, uL75, uL76, uL78, uL79, uL80, uL81, uL82, uL83, uL84, uL85, uL86, uL87, uL88, uL89, uL90) to form the 60S subunit.
- The 60S subunit also undergoes significant structural changes, with the help of assembly factors, to reach its mature state.

**3. Assembly of the 90S Pre-Ribosome**
- The 40S and 60S subunits associate to form the 90S pre-ribosome.
- The 90S pre-ribosome is a transient complex, and its formation is crucial for the initiation of translation.
- Various assembly factors, such as nucleolin, fibrillarin, and Nop56, play roles in the assembly of the 90S pre-ribosome.
- The 90S pre-ribosome undergoes further maturation and structural changes within the nucleolus.

**4. Maturation into the Functional 80S Ribosome**
- The 90S pre-ribosome undergoes a series of rearrangements and conformational changes, mediated by various proteins and enzymes, leading to the formation of the mature 80S ribosome.
- This process involves the release of assembly factors, the recruitment of additional proteins, and the refinement of the ribosomal structure.

**5. Export to the Cytoplasm**
- The mature 80S ribosome is then exported from the nucleus into the cytoplasm, where it will participate in protein synthesis.

The 90S preribosome assembly is a tightly regulated process essential for cellular viability. It ensures that the ribosome, the protein synthesis machinery, is properly assembled and functional. This process is intricately linked to the cell's growth, development, and overall health.'
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Proteins (1)

Compounds (2)