An Off-Pathway Folding Intermediate of an Acyl Carrier Protein Domain Coexists with the Folded and Unfolded States under Native Conditions**

A protein can exist in multiple states under native conditions and those states with low populations are often critical to biological function and self-assembly. To investigate the role of the minor states of an acyl carrier protein, NMR techniques were applied to determine the number of minor states and characterize their structures and kinetics. The acyl carrier protein from Micromonospora echinospora was found to exist in one major folded state (95.2%), one unfolded state (4.1%), and one intermediate state (0.7%) under native conditions. The three states are in dynamic equilibrium and the intermediate state very likely adopts a native-like structure and is an off-pathway folding product. The intermediate state may mediate the formation of oligomers invitro and play an important role in the recognition of partner enzymes invivo.