Mares-Mejia, I. | Search bioRxiv

bioRxiv2025

Molecular basis of noncanonical complement C3 activation by histamine

For fifty years the tick-over mechanism has been considered responsible for priming the activation of the complement systems alternative pathway through the reaction of a nucleophilic water molecule with C3 yielding C3(H2O), even though the exclusivity of this role has been challenged by the existence of extrinsic proteases that can cleave circulating C3 into C3b. Here we show that the biogenic amine histamine can activate C3 by reacting with the internal thioester bond yielding a novel species that is equivalent to C3(H2O), which we have called C3h. Histamine activation of C3 occurs significantly faster than the water-mediated tick-over reaction, leading to the accelerated release of the C3a anaphylatoxin moiety, contributing to inflammation. Importantly, C3h can form an active C3 convertase enzyme that, together with the released C3a, can amplify complement activation and inflammatory responses. These results offer insight into the priming of the complement system activation and support the existence of direct crosstalk between histamine-releasing processes and complement activation.

biochemistry↗

bioRxiv2025

The major diagnostic VSG LiTat 1.3 of the human parasite Trypanosoma brucei gambiense is a trimer in solution

Human African trypanosomiasis (HAT) remains a significant health burden in sub-Saharan Africa, with serological diagnosis relying heavily on parasite variant surface glycoproteins (VSGs). In this study, we present evidence that LiTat 1.3 (a key VSG in the diagnosis of T. b. gambiense infections) displays a homotrimeric architecture in solution instead of the archetypal homodimeric structure expected for a VSG. This was demonstrated by adopting an integrative structural biology approach encompassing AlphaFold-based structure prediction, analytical gel filtration (AGF), size exclusion chromatography with multi-angle light scattering (SEC-MALS), and small-angle X-ray scattering (SAXS). Furthermore, the SAXS data demonstrate that the C-terminal domains of trimeric VSGs exhibit the same degree of flexibility as observed in dimeric VSGs. Hence, the biophysical characterization of LiTat 1.3 VSG adds to the limited, yet growing body of knowledge that certain VSG classes occur as homotrimers instead of homodimers. Authors SummaryHuman African trypanosomiasis (HAT) is caused by Trypanosoma brucei gambiense, a parasite transmitted by tsetse flies. To survive in the human host, these parasites cover themselves with a coat consisting of millions of identical copies of surface proteins called variant surface glycoproteins (VSGs). This VSG coat is regularly switched by the parasite to escape the immune system. Some of these VSGs, including one known as LiTat 1.3, are used in diagnostic tests to detect potentially infected patients. In our study, we discovered that, unlike most VSGs that form pairs of identical molecules (homodimers), LiTat 1.3 assembles into groups of three (homotrimers). Using structural and biophysical techniques, we showed that this trimeric form is stable in solution and retains the dynamic behavior observed in dimeric VSGs. Understanding how such structural variations arise and how they influence immune recognition may help explain why certain VSGs, like LiTat 1.3, are particularly effective in diagnosis and could ultimately guide the development of improved tools to monitor and control sleeping sickness.

microbiology↗