Lon, an ATP-dependent AAA
protease, recognizes and degrades many different substrates, including the RcsA and SulA regulatory proteins. More than a decade ago, the E240K mutation in the N domain of Lon was shown to prevent degradation of RcsA but not SulA
. Here, we characterize the biochemical properties of the E240K mutant
and present evidence that the effects of this mutation are complex. For example, Lon
exists almost exclusively as a dodecamer, whereas wild-type Lon equilibrates between hexamers and dodecamers. Moreover, Lon
displays degradation defects
that do not correlate in any simple fashion with degron identity, substrate stability, or dodecamer formation. The Lon sequence segment near residue 240 is known to undergo nucleotide-dependent conformational changes, and our results suggest that this region may be important for coupling substrate binding with allosteric activation of Lon protease and ATPase activity.