The crystal structure and localization of Trypanosoma brucei invariant surface glycoproteins suggest a more permissive VSG coat in the tsetse-transmitted metacyclic stage

Trypanosoma brucei spp. develop into mammalian-infectious metacyclic trypomastigotes inside the tsetse salivary glands. Besides acquiring a variant surface glycoprotein (VSG) coat, nothing is known about expression of invariant surface antigens by the metacyclic stage. Proteomic analysis of saliva from T. brucei-infected flies revealed a novel family of hypothetical GPI-anchored surface proteins herein named Metacyclic Invariant Surface Proteins (MISP). MISP are encoded by five homolog genes and share ~80% protein identity. The crystal structure of MISP N-terminus at 1.82 [A] resolution revealed a triple helical bundle that shares key features with other trypanosome surface proteins. However, molecular modelling combined with live fluorescent microscopy suggest that MISP N-termini are extended above the metacyclic VSG coat, exposing immunogenic epitopes. Collectively, we suggest that the metacyclic cell surface architecture appears more permissive than bloodstream forms in terms of expression of invariant GPI-anchored glycoproteins, which could be exploited for the development of novel vaccines against African trypanosomiases.