Crystal structure of the native plasminogen reveals an activation‐resistant compact conformation

Summary.  Background:  Plasminogen is the zymogen form of plasmin and the precursor of angiostatin. It has been implicated in a variety of disease states, including thrombosis, bleeding and cancers. The native plasminogen, known as Glu-plasminogen, contains seven domains comprising the N-terminal peptide domain (NTP), five kringle domains (K1–K5) and the C-terminal serine protease domain (SP). Previous studies have established that the lysine binding site (LBS) of the conserved kringle domains plays a crucial role in mediating the regulation of plasminogen function. However, details of the related conformational mechanism are unknown. Objectives:  We aim to understand in more detail the conformational mechanism of plasminogen activation involving the kringles. Methods:  We crystallized the native plasminogen under physiologically relevant conditions and determined the structure at 3.5 A resolution. We performed structural analyses and related these to the literature data to gain critical understanding of the plasminogen activation. Results and conclusions:  The structure reveals the precise architecture of the quaternary complex. It shows that the Glu-plasminogen renders its compact form as an activation-resistant conformation for the proteolytic activation. The LBSs of all kringles, except K1, are engaged in intra-molecular interactions while only K1-LBS is readily available for ligand binding or receptor anchorage. The structure also provides insights into the interactions between plasminogen and α2-antiplasmin, the primary physiological inhibitor of plasmin. Furthermore, the data presented explain why a conformational transition to the open form is necessary for plasminogen activation as well as angiostatin generation, and provide a rationale for the functional hierarchy of the different kringles.