Spontaneous conformational change in serpins. (a) Structure of native SERPINE1 (PDB code 1B3K) . The RCL is in magenta and strand s1c of β-sheet C is in yellow. (b) The structure of latent SERPINE1 (PDB code 1DVN) [53,97], which can form by spontaneous conversion from the native protein. The RCL (magenta) is inserted into β-sheet A. In order to enable full insertion of the RCL, s1C of β-sheet C (pale yellow) has peeled off. In addition, conformational change in the strands s3C and s4C (pale green) is indicated. (c) Structure of SERPINE1 (blue) in complex with the somatomedin B domain (green) of vitronectin (PDB code 1OC0) . The interaction with vitronectin locks SERPINE1 in the native, active conformation.