Formation of high-order oligomers is required for functional bioactivity of an African bat henipavirus surface glycoprotein

Hendra virus (HeV) and Nipah virus (NiV) are highly pathogenic henipaviruses originating from fruit bats in Australia and Asia that can cause severe infections in livestock and humans. In recent years, also African bat henipaviruses were identified at the nucleic acid level. To assess their potential to replicate in non-bat species, several studies were performed to characterize the two surface glycoproteins required for virus entry and spread by cell-cell fusion. It has been shown that surface expression and fusion-helper function of the receptor-binding G protein of Kumasi virus (KV), the prototypic Ghanaian bat henipavirus, is reduced compared to other non-African henipavirus G proteins. Immunostainings and pulse-chase analysis revealed a delayed export of KV G from the ER. As defects in oligomerization of viral glycoproteins can be responsible for limited surface transport thereby restricting the bioactivity, we analyzed the oligomerization pattern of KV G. In contrast to HeV and NiV whose G proteins are known to be expressed at a dimer-tetramer ratio of 1:1, KV G almost exclusively formed stable tetramers or higher oligomers. KV G also showed less stringent requirements for defined stalk cysteines to form dimers and tetramers. Interestingly, any changes in the oligomeric forms negatively affected the fusion-helper activity although surface expression and receptor binding was unchanged. This clearly indicates that the formation of mostly higher oligomeric KV G forms is not a deficiency responsible for ER retention, but is rather a basic structural feature essential for the bioactivity of this African bat henipavirus glycoprotein.