To accomplish the task of characterizing site-specific glycosylation of the trimer Spike of SARS-CoV-2 and the host receptor ACE2, we began by expressing and purifying a stabilized, soluble trimer Spike glycoprotein mimetic immunogen (that we define here and forward as S, [Yu et al., 2020]) and a soluble version of the ACE2 glycoprotein from a human cell line.