Periplasmic chaperone Skp
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-helics 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. Outer Membrane Proteins (Omps) are proteins that reside and function in the outer membrane of bacteria. Omps are synthesized in the cytoplasm and need to pass through two more membranes, the periplasm and the peptitoglycan layer, before they are inserted into the outer membrane. Skp is a periplasmic chaperone in Escherichia coli that interacts with Omps while they travel through the periplasm and protect then from misfolding and aggregation.
The structure above is that of the Skp protein. It is a trimer with each monomer having a beta-sheet domain and arched alpha-helices. The beta-domains of the 3 monomers associate with each other to form the base of the trimer. The alpha-helices are flexible and extend out of the trimer base and approach each other at distal end to form a 'three-pronged grasping forceps'. The authors propose that such a structure is conducive for binding Omps and protecting them during their transit through the periplasm.
Protein Data Bank (PDB)
Korndorfer, I.P. Dommel, M.K. Skerra, A.; "Structure of the periplasmic chaperone Skp suggests functional similarity with cytosolic chaperones despite differing architecture."; Nat.Struct.Mol.Biol.; (2004) 11:1015-1020 PubMed:15361861.
author: Ashwini Patil