Tập tin:GroESL.jpg

a, Space-filling model of the GroEL–GroES complex. GroEL consists of two rings, each made of seven ATPase subunits arranged back to back, whereas GroES consists of a single dome-shaped ring of seven smaller subunits. The nucleotide-binding abilities of the two GroEL rings are mutually exclusive (that is, only one ring at a time can bind either ATP or ADP). In the presence of ATP or ADP, GroES binds to the same ring that binds the nucleotide, creating a cage at that end. (Reprinted from ref. 10.) b, A single, partly folded polypeptide chain released from the ribosome binds to hydrophobic residues exposed at the end of GroEL uncapped by GroES. Binding of GroES and ATP to that end triggers release of the chain into the newly created cage because GroES competes for binding sites with the partly folded chain. The chain has 10–15 seconds to fold inside this cage, because this is the time it takes for the ATP to be hydrolysed to ADP by the ring that contains the folding protein. The ADP then unbinds, allowing ATP to bind to the opposite ring. This binding triggers the release of GroES and of the folded polypeptide from the ring containing the cage. If the released chain has not internalized all its hydrophobic residues, it is likely to be rebound by the same ring. (Revised from ref. 1.)