|Résumé: ||Rhomboids are unique membrane proteins that use a serine protease hydrolysis mechanism to cleave a transmembrane substrate within the lipid bilayer. This remarkable proteolytic activity is achieved by a core domain comprised of 6 transmembrane segments that form a hydrophilic cavity submerged in the membrane. In addition to this core domain, many rhomboids also possess aqueous domains of varying sizes at the N- and/or C-terminus, the sequences of which tend to be rhomboid-type specific. The functional role of these extramembranous domains is generally not well understood, although it is thought that they may be involved in regulation of rhomboid activity and specificity. While extramembranous domains may be important for rhomboid activity, they are absent in all x-ray crystal structures available. For this reason, we have focused on uncovering the structural and functional relationship between the rhomboid cytoplasmic domain and its catalytic transmembrane core.
To investigate the structure and function of the bacterial rhomboid cytoplasmic domain, full-length rhomboids from Escherichia coli and Pseudomonas aeruginosa were studied using solution nuclear magnetic resonance (NMR) spectroscopy, mutation and activity assays. The P. aeruginosa rhomboid was purified in a range of membrane-mimetic media, evaluated for its functional status in vitro and investigated for its NMR spectroscopic properties. Results from this study suggested that an activity-modulating interaction might occur between the catalytic core transmembrane domain and the cytoplasmic domain. Further investigation of this hypothesis with the E. coli rhomboid revealed that protease activity relies on a short but critical sequence N-terminal to the first transmembrane segment. This sequence was found to have a direct impact on the rhomboid active site, and should be included in future structural studies of this catalytic domain.
The structure of the cytoplasmic domain from the E. coli rhomboid was also determined by solution NMR. We found that it forms slowly-exchanging dimers through an exchange of secondary structure elements between subunits, commonly known as three-dimensional domain swapping. Beyond this rare example of domain swapping in a membrane protein extramembranous domain, we found that the rate of exchange between monomeric and dimeric states could be accelerated by transient interactions with large detergent micelles with a phosphocholine headgroup, but not by exposure to other weakly denaturing conditions. This novel example of micelle-catalyzed domain swapping interactions raises the possibility that domain swapping interactions might be induced by similar interactions in vivo. Overall, the results of this thesis have identified detergent conditions that preserve the highest level of activity for bacterial rhomboids, defined the minimal functional unit beyond what had been identified in available x-ray crystal structures, and characterized a novel micelle-catalyzed domain-swapping interaction by the cytoplasmic domain.|