bacteriophage T4 fibritin foldon trimerization sequence

So I'm still trying to figure this out, but one preliminary thought is that this S-protein might be a modified version, which forces the desired conformation of the spike (either 'open' or 'closed') to be used in vaccines.

(The original Spike might change conformation somewhat uncontrollably, so might not be best suited for S-protein based vaccine. )