Ribosomal chaperon trigger factor complexed with 50S ribosome
Escherichia coli (bacteria)
In order to function correctly, proteins need to fold into their correct tertiary structure. The folding process starts as soon as the partial polypeptide chain (that will eventually become the protein) exits the ribosome on translation. The secondary structures such as alpha-helices and beta-sheets are formed at this stage. However, this is not the final conformation of the protein since its domains are still flexible. The protein has to go through various changes in the orientation of its secondary structures and side chains, before it achieves its final conformation. Some of the intermediate conformations that the protein goes through are susceptible to aggregation or incorrect final conformation. The folding of such proteins is aided by a special class of proteins called chaperones. Chaperones protect partially folded polypeptides from misfolding or aggregating. They assist biological systems in achieving proper folding of proteins. Ribosome-associated chaperones help in the proper folding of nascent polypeptide chains that exit the ribosomal tunnel during protein synthesis.
The structure shown here is that of a dimer of the trigger factor, a ribosome-associated chaperone in Escherichia coli. The trigger factor binds to the ribosome as a monomer. The trigger factor consists of a tail at the N-terminus, a head and two arms at the C-terminus. The tail binds the ribosome near the exit tunnel and is followed by the two predominantly alpha-helical arms which form the core of the protein. The protein has a cradle-like conformation with a highly hydrophobic inner surface. The authors propose that the cradle of the trigger factor covers the ribosomal exit tunnel and binds to the exiting nascent polypeptide chain, thus protecting it and helping it acquire its native conformation.
Protein Data Bank (PDB)
Ferbitz, L. Maier, T. Patzelt, H. Bukau, B. Deuerling, E. Ban, N.; "Trigger Factor in Complex with the Ribosome Forms a Molecular Cradle for Nascent Proteins"; Nature; (2004) 431:590-596 PubMed:15334087.
author: Ashwini Patil