Marty Rechsteiner's PubMed Literature Search
Besides the RC there are two other proteins that bind the ends of the 20S proteasome and activate peptide hydrolysis. One is a heptamer of 30K subunits called PA28; the other is a large single polypeptide chain called PA200. We study these two activators as well as a proteasome adaptor called Ecm29 that we propose targets the 26S proteasome to sites of increased proteolysis within eukaryotic cells. Over the past six years, we have cloned and expressed 9 of the RC subunits. We are now using Far Western blotting and in vitro assembly reactions to determine the relative positions of subunits within the RC. Another important goal is to assign functions to each subunit. Six RC subunits belong to a family of ATPases approximately 400 amino acids in length. Although the central regions in the ATPases are highly conserved, the N-terminal and C-terminal sequences are divergent. We have constructed a series of chimeric ATPases to determine whether the divergent regions recognize specific substrates. Several years ago we found that one RC subunit (S5a) binds polyubiquitin chains. We recently discovered that a dimer of two other subunits also binds polyUb chains. We are characterizing the polyUb binding site(s) within these proteins. In addition, we are examining the peptide binding properties of RC subunits because some RC components may bind proteasome extensions to assemble the 26S proteasome and others may bind unfolded regions in proteolytic substrates.
There is a second form of proteasomes. In 1992, we discovered a potent proteasome activator, called REG, that is induced by interferon-* and is thought to play a role in antigen presentation. REG is composed of seven subunits arranged as a ring capable of binding to both ends of the cylindrical proteasome. Our working hypothesis is that REG couples the proteasome to specific cellular structures. We are using 2-hybrid screens and GST-REG chimeras to identify such components.