Structural and Functional Study on the Interaction of Human Plasminogen and Protein H from Haemophilus Influenzae Type F

Abstract

Haemophilus influenzae serotype f (Hif) resides in human respiratory tract, is an emerging pathogen causes invasive diseases. Recently, we annotated the whole genome of an invasive Hif isolate and showed that Hif simultaneously interacts with factor H and vitronectin by using the surface protein H (PH). Here, upon screening with various human plasma proteins, we found that PH can also interact with plasminogen, an important coagulation regulator of the blood. Plasminogen bound at a bacterial surface can be converted into plasmin and degrades complement factors such as C3b and C5 and thus suppresses the host innate immunity. In order to study the structural interaction between PH and human plasminogen, we cloned the lph gene encoding the protein PH in pETM30 vector and optimized the expression and purification of untagged recombinant PH. The dissociation constant (Kd) of the interaction between recombinant PH and plasminogen was 2.7 μM as measured by Biolayer interferometry and Microscale thermophoresis. We demonstrate that Hif can bind plasminogen, which is converted to plasmin that degrades complement factor C3b, C5 and blood clotting factor fibrinogen. We are now crystallizing PH, and the PH-plasminogen complex. Our study will spot light on deeper understanding of Hif virulence.