The GroEL double-ring cylinder is a compact structure that in the presence of nucleotides is resistant to proteolysis by proteinase K. Only the last 16 carboxy-terminal residues of each subunit are cleaved (1,2), as determined by mass spectrometry. These residues are part of a Gly-Met-rich tail, which is buried inside the cylindrical chaperonin cavity at the level of the equatorial subunit domains (3). The truncated GroEL (ELAC), in which these 16 residues are missing, forms an intact oligomer, can bind nucleotides, GroES, and unfolded proteins, and is functional as a chaperonin in mediating protein refolding. A notable difference, however, is its reduced ATPase activity, which is only 25% that of wild-type GroEL (1). Two properties of the GroEL oligomer permit proteinase K to attack the carboxy-termini of the chaperonin subunits. First, owing to their flexibility, the tails can reach out of the cylinder cavity, as shown by the fact that a carboxy-terminally His-tagged GroEL is able to bind to Ni-NTA affinity columns (4). Second, proteinase K (29 kDa) is small enough to enter the cavity and to exert its proteolytic activity there.