The N terminus of the head protein of T4 bacteriophage directs proteins to the GroEL chaperonin.

The head protein of T4 bacteriophage requires the GroEL chaperonin for its insertion into a growing T4 head. Hundreds of thousands of copies of this protein must pass through the chaperonin in a limited time later in infection, indicating that the protein must use GroEL very efficiently and may contain sequences that bind tightly to GroEL. We show that green fluorescent protein (GFP) fused to the N terminus of the head protein can fold at temperatures higher than those at which the GFP protein can fold well by itself. We present evidence that this folding is promoted by the strong binding of N-terminal head protein sequences to GroEL. This binding is so strong that some fusion proteins can apparently deplete the cell of the GroEL needed for other cellular functions, altering the cellular membranes and slowing growth.