Protein transduction in human cells is enhanced by cell-penetrating peptides fused with an endosomolytic HA2 sequence

► DNA plasmids containing coding sequences of CPP, HA2, and RFP were constructed. ► Endocytosis is the major route for cellular uptake of CPP-HA2-tagged RFPs. ► HA2 tag of the CPP-HA2 fusion protein improved cytosolic uptake in human A549 cells. ► CPP-HA2-tagged RFP fusion proteins did not display cytotoxicity. ► The CPP-HA2 tag could be an efficient and safe carrier of bioactive molecules.