Traditionally vaccines are made of inactivated, or live-attenuated pathogens, however modern vaccines are often based on a vector that expresses a single viral protein or an isolated, highly purified antigenic protein subunit. For such modern approaches a deep understanding of the protein structure, function and stability is needed to guarantee successful expression of natively folded stable protein multimers which can induce an immune response with strong neutralizing activity against the pathogen.
Advances in structural vaccinology and knowledge on the function of the viral surface proteins has resulted in structure-based designs of vaccine candidates for RSV and HIV. In both fusion proteins we have introduced stabilizing mutations in the metastable hinge loop preceding the base helix. We and others have now applied this to various other class I fusion proteins. Another conserved metastable structural element in these fusion proteins is a metastable intersubunit beta sheet composed of beta strands of the head and the fusion domain. Detachment or ‘‘splaying’’ of the head of the class I fusion proteins is expected ultimately to disrupt the intersubunit sheet and trigger refolding. We found several examples how stabilization of the intersubunit b-sheet may be a general strategy for conformationally fixing the prefusion state. These new insights can potentially be important for the design of future vaccine immunogens.