Several bacterial species recruit the complement regulators C4b-binding protein, factor H, and vitronectin, resulting in resistance against the bactericidal activity of human serum. It was recently demonstrated that bacteria also bind plasminogen, which is converted to plasmin that degrades C3b and C5. In this study, we found that a series of clinical isolates (n = 58) of the respiratory pathogen Moraxella catarrhalis, which is commonly isolated from preschool children and adults with chronic obstructive pulmonary disease (COPD), significantly binds human plasminogen. Ubiquitous surface protein A2 (UspA2) and hybrid UspA2 (UspA2H) were identified as the plasminogen-binding factors in the outer membrane proteome of Moraxella. Furthermore, expression of a series of truncated recombinant UspA2 and UspA2H proteins followed by a detailed analysis of protein-protein interactions suggested that the N-terminal head domains bound to the kringle domains of plasminogen. The binding affinity constant (KD) values of full-length UspA230–539 (amino acids 30 to 539 of UspA2) and full-length UspA2H50–720 for immobilized plasminogen were 4.8 × 10?8 M and 3.13 × 10?8 M, respectively, as measured by biolayer interferometry. Plasminogen bound to intact M. catarrhalis or to recombinant UspA2/UspA2H was readily accessible for a urokinase plasminogen activator that converted the zymogen into active plasmin, as verified by the specific substrate S-2251 and a degradation assay with fibrinogen. Importantly, plasmin bound at the bacterial surface also degraded C3b and C5, which consequently may contribute to reduced bacterial killing. Our findings suggest that binding of plasminogen to M. catarrhalis may lead to increased virulence and, hence, more efficient colonization of the host.
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机译：几种细菌会募集补体调节剂C4b结合蛋白，H因子和玻连蛋白，导致对人血清杀菌活性的抵抗力。最近证明细菌还结合纤溶酶原，纤溶酶原被转化为可降解C3b和C5的纤溶酶。在这项研究中，我们发现一系列呼吸道病原体卡他莫拉菌的临床分离株（ n = 58），通常从学龄前儿童和患有慢性阻塞性肺疾病（COPD）的成年人中分离出来结合人纤溶酶原。普遍存在的表面蛋白A2（UspA2）和杂种UspA2（UspA2H）被鉴定为莫拉氏菌外膜蛋白质组中的纤溶酶原结合因子。此外，表达一系列截短的重组UspA2和UspA2H蛋白，然后对蛋白-蛋白相互作用进行详细分析，表明N末端头部结构域与纤溶酶原的kringle域结合。全长UspA2 30–539 （UspA2的第30至539位氨基酸）和完整的UspA2 30–539 的结合亲和常数（ K D ）值固定化纤溶酶原的长度UspA2H 50–720 分别为4.8×10 ？8 M和3.13×10 ？8 M干涉仪。尿激酶纤溶酶原激活剂可以很容易地将与粘膜炎莫拉氏菌或重组UspA2 / UspA2H结合的纤溶酶原转化为酶原，使其转化为活性纤溶酶，这一点已通过特定的底物S-2251和纤维蛋白原的降解检测得以证实。重要的是，结合在细菌表面的纤溶酶也降解了C3b和C5，因此可能有助于减少细菌的杀灭。我们的发现表明，纤溶酶原与粘膜炎莫拉氏菌的结合可能导致毒力增加，从而使宿主更有效地定殖。
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