Multi-protein assemblies orchestrate co-translational enzymatic processing on the human ribosome.

Area of Science:

• Molecular Biology
• Structural Biology
• Biochemistry

Background:

• Nascent proteins undergo enzymatic modifications immediately after translation begins.
• N-terminal methionine excision (NME) and N-terminal acetylation (NTA) are common co-translational modifications on eukaryotic 80S ribosomes.
• The coordination of these enzymatic processes during active translation remains unclear.

Purpose of the Study:

• To elucidate the structural mechanisms coordinating N-terminal methionine excision (NME) and N-terminal acetylation (NTA) on the human 80S ribosome.
• To investigate how Methionine Aminopeptidases (MAP1, MAP2) and N-Acetyl-Transferase A (NatA) interact with the ribosome and each other.
• To understand the role of the Nascent Polypeptide-Associated Complex (NAC) in coordinating these modifications.

Main Methods:

• Cryo-electron microscopy (cryo-EM) was used to determine the structures of multi-enzyme complexes on vacant human 80S ribosomes.
• Structural analysis focused on the binding sites and interactions of NatA, MAP1, MAP2, and NAC with the ribosome.
• The study examined the compatibility of these factors with the ribosomal polypeptide tunnel exit (PTE) and other ribosome-associated factors (RAFs).

Main Results:

• Two distinct structural assemblies reveal two pathways for NME-NTA coordination on the 80S ribosome.
• These assemblies form independently of nascent polypeptide chains.
• NatA utilizes a distal binding site that accommodates MAP1/MAP2 and most RAFs, while MAP2 obstructs the PTE, preventing NAC and MAP1 recruitment.

Conclusions:

• The study provides a structural framework for the coordinated orchestration of NME and NTA during protein biogenesis.
• NatA can dynamically assemble with MAP1 via NAC, suggesting a coordinated pathway.
• MAP2's interaction with the PTE highlights an alternative, incompatible pathway for NME-NTA coordination.