The naturally selected mutations S477N, E484K, and N501Y of the Spike protein RBD, which show higher infectivity, were selected by yeast surface display affinity maturation already in the first round, giving rise to the South-African, E484K, N501Y, and British variants that bind ACE2 13 and 3.5-fold tighter than RBD-WT. Following three additional rounds of yeast display selection resulted in 600-fold tighter binding in comparison to RBD-WT.
