A different two-step mechanism has been suggested and in this case  the virions binds to a receptor on the target host cell surface through its S1 subunit and the spike  is cleaved by host proteases and then it is expected the fusion at low pH between  viral and host target membranes  via S2 subunit. Finally, the viral genetic material a single stranded RNA is fully released into the cytoplasm. There takes place the replication and transcription processes which are mediated by the so-called replication/transcription complex (RTC)