Polyarginine enters cells more efficiently than other polycationic homopolymers

Homopolymers or peptides containing a high percentage of cationic amino aci ds have been shown to have a unique ability to cross the plasma membrane of cells, and consequently have been used to facilitate the uptake of a varie ty of biopolymers and small molecules. To investigate whether the polycatio nic character of these molecules, or some other structural feature, was the molecular basis for the effect, the ability of a variety of homopolymers t o enter cells was assayed by confocal microscopy and flow cytometry. Polyme rs of L- or D-arginine containing six or more amino acids entered cells far more effectively than polymers of equal length composed of lysine, ornithi ne and histidine. Peptides of fewer than six amino acids were ineffective. The length of the arginine side-chain could be varied without significant l oss of activity. These data combined with the inability of polymers of citr ulline to enter cells demonstrated that the guanidine headgroup of arginine was the critical structural component responsible for the biological activ ity. Cellular uptake could be inhibited by preincubation of the cells with sodium azide, but not by low temperature (3 degreesC), indicating that the process was energy dependent, but did not involve endocytosis.