Native human Glu-plasminogen (Glu1-Asn791) was previously shown to have a radius of gyration of 39 Åand a shape best described by a prolate ellipsoid [Mangel, W. F., Lin, B., & Ramakrishnan, V. (1990) Science 248, 69–73]. Upon occupation of a weak lysine-binding site, the shape reversibly changes to that best described by a Debye random coil with a radius of gyration of 56 Å. Conversion from the closed to the open form is not accompanied by any change in secondary structure, hence the closed conformation is formed by interaction between domains, the five kringles and the protease domain, and this is abolished upon conversion to the open form. Here we analyzed by small-angle neutron scattering the conformations of human Lys-plasminogen (Lys78-Asn791) and the fragment K1-3 that contains the first three kringles of plasminogen (Tyr80-Val338 or Tyr80-Val354 The shape of Lys-plasminogen was best described by a Debye random coil with a radius of gyration of 51 Å, and occupation of its lysine-binding sites by 6-aminohexanoic acid did not dramatically alter its conformation. Thus Lys-plasminogen was in the open form, similar to that of Glu-plasminogen with its lysine-binding sites occupied. The fragment K1-3 in the absence or presence of 6-aminohexanoic acid had a shape best described equally either by an elongated prolate ellipsoid or by a Debye random coil, with a radius of gyration of 29 Å. Our model for the two forms of plasminogen is that, in the closed form, domain interaction generates a compact, almost globular, structure. Upon ligand binding or removal of the NH2-terminal peptide (Glu1-Lys77), domain interaction is abolished, but the domains remain intact. Flexibility in the polypeptide chains between the domains then gives rise to the extended and flexible structure that is characteristic of the open form. The structural differences between the closed and open forms of plasminogen are discussed in terms of functional differences relevant to the regulation of plasminogen activation.
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