The processive protease ClpAP translocates its substrate in discrete steps

BIOL 254

Laura D. Jennings, lauraj@mit.edu1, Desmond S. Lun2, Muriel Médard2, and Stuart Licht, lichts@mit.edu1. (1) Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139, (2) Laboratory for Information and Decision Systems, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139
Processive proteases such as the bacterial Clp proteases and the mammalian proteasome serve two roles within the cell: degradation of misfolded proteins and (in mammalian cells) generation of antigenic peptides. Complete degradation of proteins requires processive proteolysis while antigen formation requires homogeneous pools of peptides of optimal length for epitope presentation (8-9 residues). Analysis of the ClpAP peptide product size distribution reveals a mechanism that could help optimize both the degradative and signaling functions of these proteases. Both size-exclusion chromatography and MALDI mass spectrometry unveil a size distribution with pools of peptides at intervals corresponding to discrete steps in translocation. Our data support a model in which ClpAP translocates its substrates in 5-7 residue segments and, following each translocation step, the enzyme partitions between further translocation and proteolytic cleavage. This evidence for stepwise translocation indicates that ClpAP may share an unexpected mechanistic commonality with related ATPase-driven motor proteins. Numerical simulations show that stepwise translocation enhances the ability of processive proteases to generate specific products of a defined length; use of this mechanism by mammalian proteasomes might account for the observed efficiency of epitope production.
 

Frontiers in Chemical Biology
5:00 PM-7:00 PM, Wednesday, August 22, 2007 BCEC -- Exhibit Hall - B2, Poster

Division of Biological Chemistry

The 234th ACS National Meeting, Boston, MA, August 19-23, 2007