The time-specific requirement of Rho proteins for the S phase progression of vascular endothelial cells was determined by reversibly introducing inhibitor proteins with a cell-penetrating peptide. We found evidence of the reversibility of protein transduction. The removal of extracellular protein caused the transduced protein to decay in a manner sensitive to low temperatures. The time required for a 50% decay correlated with the protein size. The time-specific transduction of the inhibitor proteins uncovered a critical period requiring Rho proteins in the G1-S transition phase. Reversible protein transduction may thus be a powerful tool to investigate the time-specific role of signaling proteins.
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