| Comments |
p.275 left column bottom paragraph to p.276 left column top paragraph:"How does the unfolding/degradation of protein substrates by Lon compare with these activities for other AAA+ proteases? One way to rank the activities of different enzymes is to compare Vmax, the rate of degradation at substrate saturation using proteins for which unfolding is rate limiting. Table II lists Vmax values for Lon, ClpXP, and ClpAP degradation of titinI27 and GFP substrates with recognition tags at the C-terminus or the N-terminus. For C-tagged titinI27 substrates, Lon displayed the highest Vmax, ClpXP was next highest, and ClpAP had no detectable activity (Table II). For N-tagged titinI27 proteins, ClpAP had the highest Vmax, and Lon and ClpXP were about 10-fold less active. For C-tagged GFP, the order of unfolding/degradation activities was ClpAP>ClpXP>Lon. For N-tagged GFP, Lon and ClpXP were essentially inactive, whereas ClpAP was very active. These comparisons indicate that the ranking of enzyme unfolding activity changes as a function of the substrate and position of the degradation tag. Moreover, [investigators] previously found that Vmax for degradation of titinI27 from the C-terminus varied ~sixfold, depending on the degron employed. Thus, although Lon appears to be a better unfoldase for C-tagged titinI27 substrates than ClpXP and substantially more active than ClpAP (Table II), these rankings might be degron dependent." mDHFR=mouse dihydrofolate reductase. Degron=a specific sequence of amino acids in a protein that directs the starting place of degradation [wikipedia]. |