Fibronectin type I repeats are one of the three repeats found in the fibronectin protein.Fibronectin is a plasma protein that binds cell surfaces and various compoundsincluding collagen, fibrin, heparin, DNA, and actin. Type I domain (FN1) is approximately40 residues in length. Four conserved cysteines are involved in disulphide bonds. The 3Dstructure of the FN1 domain has been determined [, , ]. It consists of two antiparallelβ-sheets, first a double-stranded one, that is linked by a disulphide bond to atriple-stranded β-sheet. The second conserved disulphide bridge links the C-terminaladjacent strands of the domain.In human tissue plasminogen activator chain A the FN1 domain together with thefollowing epidermal growth factor (EGF)-like domain are involved infibrin-binding []. It has been suggested that these two modules form a single structuraland functional unit []. The two domains keep their specific tertiary structure, but interactintimately to bury a hydrophobic core; the inter-module linker makes up the third strand ofthe EGF-module's major β-sheet.
Tissue-type plasminogen activator is a serine peptidase belonging to MEROPS peptidase family S1 (chymotrypsin family, clan PA(S)), subfamily S1A.The fibrinolytic system is the mechanism by which blood clots are dissolved via the plasmin-mediated degradation of fibrin into soluble end products. The system is activated by urokinase-type plasminogen activator (uPA; ) or by tissue-type plasminogen activator (tPA; ; this family), which are enzymes that convert the inactive proenzyme plasminogen to the active protease plasmin [].tPA is mainly involved in the activation of circulating plasminogen, while uPA activates cell-bound plasminogen after binding to a specific uPA receptor (uPAR).Following the signal sequence and propeptide regions, tPA typically contains a fibronectin type I (FN1) domain, an EGF-type domain, two kringle domains and a trypsin domain. However, there are members missing FN1 or EGF or one of the kringles.This entry also includes salivary plaminogen activators from the vampire bat (Desmodus rotundus).
