Clp is a barrel-shaped hetero-oligomeric ATP-dependent protease comprising a hexameric ATPase (ClpX or ClpA) that unfolds protein substrates and translocates them into the central chamber of the tetradecameric proteolytic component (ClpP) where they are degraded processively to short peptides. Chamber access is controlled by the N-terminal 20 residues (for Escherichia coli) in ClpP that prevent entry of large polypeptides in the absence of the ATPase subunits and ATP hydrolysis. Remarkably, removal of 10-17 residues from the mature N-terminus allows processive degradation of a large model unfolded substrate to short peptides without the ATPase subunit or ATP hydrolysis; removal of 14 residues is maximal for activation. Furthermore, since the product size distribution of Delta 14-ClpP is identical to ClpAP and ClpXP, the ATPases do not play an essential role in determining this distribution. Comparison of the structures of Delta 14-ClpP and Delta 17-ClpP with other published structures shows R15 and S16 are labile and that residue 17 can adopt a range of rotomers to ensure protection of a hydrophobic pocket formed by 119, R24 and F49 and maintain a hydrophilic character of the pore. (C) 2008 Elsevier Inc. All rights reserved.