In contrast, only closed S trimers have been detected for the four other human-infecting coronaviruses: HCoV-NL63, HCoV-OC43, HCoV-HKU1 and HCoV-229E. As HCoV-NL63 and HCoV-229E are known to engage protein receptors through SB, trimer opening is also expected to occur to expose their RBMs that are otherwise buried at the interface between protomers in the closed S trimers. Regardless of the nature of the receptor and the location of the receptor-binding domains, removal of the trimeric S1 crown is expected to be necessary for all coronaviruses to allow the large scale S2 conformational changes leading to fusion of the viral and host membranes.