Cotranslational folding of globin

Area of Science:

• Molecular Biology
• Biochemistry
• Protein Synthesis

Background:

• Heme is crucial for globin function.
• The process of heme incorporation into nascent globin chains is not fully understood.
• Investigating the minimal chain length for heme binding can elucidate early folding events.

Purpose of the Study:

• To determine the minimal length of nascent ribosome-bound globin chains capable of efficient heme binding.
• To investigate the relationship between nascent chain length, folding, and heme interaction.
• To explore the possibility of cotranslational folding in globin synthesis.

Main Methods:

• Wheat germ cell-free translation system.
• Synthesis of truncated globin mRNA to produce nascent polypeptides of predetermined sizes.
• Use of radiolabeled hemin ([3H]hemin) and methionine ([35S]methionine) for detection.
• Analysis via sucrose gradient centrifugation and puromycin reaction.

Main Results:

• Ribosome-bound N-terminal alpha-globin fragments of 140, 100, and 86 amino acid residues efficiently bind heme.
• Fragments of 75, 65, and 34 amino acid residues showed significantly weaker or non-specific affinity to heme.
• A nascent chain length of 86 amino acids appears sufficient for spatial structure formation enabling heme interaction.

Conclusions:

• Efficient heme binding to nascent globin chains requires a minimum length of approximately 86 amino acid residues.
• Heme binding may either be facilitated by pre-existing structure in the nascent chain or promote the formation of proper tertiary structure.
• These findings strongly suggest cotranslational folding of globin chains during protein synthesis.