In Gram-negative bacteria, the outer membrane contains primarily β -barrel transmembrane proteins and lipoproteins. The insertion and assembly of β -barrel outer-membrane proteins (OMPs) is mediated by the β -barrel assembly machinery (BAM) complex, the core component of which is the 16-stranded transmembrane β -barrel BamA. Recent studies have indicated a possible role played by the seam between the first and last β -barrel strands of BamA in the OMP insertion process through lateral gating and a destabilized membrane region. In this study, we have determined the stability and dynamics of the lateral gate through over 12.5 μs of equilibrium simulations and 4 μs of free-energy calculations. From the equilibrium simulations, we have identified a persistent kink in the C-terminal strand and observed spontaneous lateral-gate separation in a mimic of the native bacterial outer membrane. Free-energy calculations of lateral gate opening revealed a significantly lower barrier to opening in the C-terminal kinked conformation; mutagenesis experiments confirm the relevance of C-terminal kinking to BamA structure and function.