Protamine as an efficient membrane-translocating peptide

Protamine, a mixture of positively charged proteins from salmon roe used in diverse pharmaceutical applications, was reacted with the N-hydroxysuccinimide ester of tetramethylrhodamine to yield tetramethylrhodamine-labeled protamines (Pro(Rh)) containing one mole of fluorochrome per mole of protein. The internalization of tetramethylrhodamine-labeled protamine (Pro(Rh)) and the fluorescein-labeled tat peptide (Tat(Fl)) showed a similar dependence on time and concentration. Pro(Rh) and Tat(Fl) showed strong nuclear localizations, evident with both live cells and fixed cells co-stained with DAPI, a nuclear stain. The loss of fluorescence when cells were loaded with Pro(Rh) or Tat(Fl) was similar, further supporting a strong similarity between these two materials. Finally, when Pro(Ph) was covalently attached to the amino-CLIO nanoparticle, the cellular uptake of the nanoparticle was greatly enhanced. All experiments were performed with HeLa and CaCo-2 cells with similar results. These observations imply that protamine, a protein in regular clinical use, might be used for the design of novel membrane translocating/nuclear localizing pharmaceuticals whose development was initiated with other membrane-translocating peptides. In addition, the fluorescent protamines developed here might be used to further our understanding of this important pharmaceutical.