Structural and antigenic characterization of Babesia bovis HAP2 domains

Authors: RAHMAN, RAIHAN - Washington State University; ALZAN, HEBA - Washington State University; Laughery, Jacob; Bastos, Reginaldo; Ueti, Massaro; SUAREZ, CARLOS - Retired ARS Employee

Submitted to: Scientific Reports
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/19/2025
Publication Date: 3/5/2025
Citation: Rahman, R.S., Alzan, H.F., Laughery, J.M., Bastos, R.G., Ueti, M.W., Suarez, C.E. 2025. Structural and antigenic characterization of Babesia bovis HAP2 domains . Scientific Reports. (15):7781. https://doi.org/10.1038/s41598-025-91359-4.
DOI: https://doi.org/10.1038/s41598-025-91359-4

Interpretive Summary: Bovine babesiosis caused by B. bovis causes enormous food and economic losses around the world. The existing resources for managing this disease are limited and have pitfalls; therefore, the introduction of new strategies is urgently needed. B. bovis reproduces sexually in the midgut of its tick vector. We previously demonstrated that HAP2, a well-conserved ancient protein, plays a crucial role in the gamete fusion of this parasite and is a strong candidate for developing transmission-blocking vaccines. Hereby we dissected the structure of the HAP2 protein of B. bovis and defined 3 surface exposed domains that are recognized by antibodies in transmission-protected bovines that may be crucial for its function and that can be the target of novel and more refined vaccine approaches. The findings in this study will facilitate further investigations of the HAP2 proteins of other Babesia parasites responsible for bovine babesiosis and on determining what HAP2 domains are able to elicit transmission blocking immunity when used in cattle immunization trials.

Technical Abstract: The tick-borne apicomplexan parasite Babesia bovis causes bovine babesiosis which leads to enormous food and economic losses around the world. The existing resources to manage this disease are limited and have pitfalls, therefore, introduction of new strategies is urgently needed. B. bovis reproduces sexually in the midgut of its tick vector. HAP2, a well conserved ancient protein, plays a crucial role in the gamete fusion of this parasite and is a strong candidate for developing transmission-blocking vaccines. We previously demonstrated that immunization of cattle with B. bovis HAP2 blocks transmission of the parasite by Rhipicephalus microplus. Understanding the conserved structural features and antigenicity of HAP2 protein and its domains will facilitate developing effective methods to control pathogen transmission. In this study, we analyzed and compared AlphaFold2-predicted 3D structure of B. bovis HAP2 with the well-characterized crystal structures of HAP2 of Chlamydomonas reinhardtii and Arabidopsis thaliana. The comparisons and structural analysis resulted in the definition of three domains' sequences, fusion loops, and disulfide bonds in the B. bovis HAP2. In addition, recombinant versions of each three predicted HAP2 domains were recognized by antibodies from HAP2 immunized and transmission-protected cattle, confirming their antigenicity. Remarkably, domain II was highly recognized compared to the other two domains. This study introduces new directions in designing novel functional assays and improved vaccine design through targeting the HAP2 protein.