MD simulations of all eight systems converged after approximately 50 ns of simulation time, in each case to structures that remained close (≤ 4.0 Å) to the starting model, as evidenced by plots of C RMSD values averaged across the latter part (50 -500 ns) of the simulations (Figure ). Comparisons of C RMSD values for the two individual domains identified that the major contribution to these values arises from the changes in the transmembrane domain rather than the catalytic domain. Consistent with our previous finding that the isolated catalytic domain behaves as a rigid, stable entity, comparison of catalytic domain structures in frames extracted from simulations of the truncated and full-length proteins (Figure ) identifies only minor differences localized to three regions: residues 219 -234 that form the N-terminus of the isolated catalytic domain; residues 414 -424 that form a surface loop that is undefined in some crystal structures (e.g. 5LRM ) and sits close to the membrane surface; and residues 298 -305 that are also positioned to contact the membrane. An overall C RMSD of 1.79 Å, however, supports our previous conclusion that the secondary and tertiary structure of the periplasmic MCR-1 catalytic domain is largely unaffected by the presence of the transmembrane domain and/or the membrane itself.