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The SARS-CoV-2 spike (S) protein is the target of vaccine design efforts to end the COVID-19 pandemic. Despite a low mutation rate, isolates with the D614G substitution in the S protein appeared early during the pandemic, and are now the dominant form worldwide. Here, we analyze the D614G mutation in the context of a soluble S ectodomain construct. Cryo-EM structures, antigenicity and proteolysis experiments suggest altered conformational dynamics resulting in enhanced furin cleavage efficiency of the G614 variant. Furthermore, furin cleavage altered the conformational dynamics of the Receptor Binding Domains (RBD) in the G614 S ectodomain, demonstrating an allosteric effect on the RBD dynamics triggered by changes in the SD2 region, that harbors residue 614 and the furin cleavage site. Our results elucidate SARS-CoV-2 spike conformational dynamics and allostery, and have implications for vaccine design.