LptA assembles into rod-like oligomers involving disorder-to-order transitions

LptA is a periplasmic protein involved in the transport of lipopolysaccharide (LPS) from the inner membrane (IM) to the outer membrane (OM) of Gram-negative bacteria. Growing evidence supports a model in which LptA assembles into oligomers, forming a physical bridge connecting IM and OM. This work investigates assembly and architecture of LptA oligomers. Circular dichroism and "native" electrospray-ionization ion-mobility mass spectrometry (ESI-IM-MS) are employed to test concentration dependence of LptA structural features and to analyze the morphology of higher-order aggregates. The results show that LptA progressively assembles into rod-like oligomers without fixed stoichiometry, and grows by an n + 1 mechanism up to at least the pentamer. The oligomerization process induces disorder-to-order transitions in the polypeptide chain. Comparison with crystallographic and computational data suggests that these conformational changes likely involve short disordered regions at the N- and C-termini of monomeric LptA. The protein response to thermal denaturation displays strong concentration dependence, indicating that oligomerization increases protein stability. LptA conformational stability can also be enhanced by in vitro LPS binding. The genesis of these fibrillar structures could be relevant for the correct transport of LPS across the bacterial periplasm.