Cytotoxic ribonucleases: the dichotomy of Coulombic forces.
Cells tightly regulate their contents. Still, nonspecific Coulombic interactions between cationic molecules and anionic membrane components can lead to adventitious endocytosis. Here, we characterize this process in a natural system. To do so, we create variants of human pancreatic ribonuclease (RNase 1) that differ in net molecular charge. By conjugating a small-molecule latent fluorophore to these variants and using flow cytometry, we are able to determine the kinetic mechanism for RNase 1 internalization into live human cells. We find that internalization increases with solution concentration and is not saturable. Internalization also increases with time to a steady-state level, which varies linearly with molecular charge. In contrast, the rate constant for internalization (t1/2 = 2 h) is independent of charge. We conclude that internalization involves an extracellular equilibrium complex between the cationic proteins and abundant anionic cell-surface molecules, followed by rate-limiting internalization. The enhanced internalization of more cationic variants of RNase 1 is, however, countered by their increased affinity for the cytosolic ribonuclease inhibitor protein, which is anionic. Thus, Coulombic forces mediate extracellular and intracellular equilibria in a dichotomous manner that both endangers cells and defends them from the potentially lethal enzymatic activity of ribonucleases.
Johnson
R Jeremy
RJ
Department of Biochemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706-1544, USA.
Chao
Tzu-Yuan
TY
Lavis
Luke D
LD
Raines
Ronald T
RT
D001696:Biomechanical Phenomena; D016923:Cell Death; D004705:Endocytosis; D005456:Fluorescent Dyes / Q000737:chemistry; D006801:Humans; D020014:K562 Cells; D007700:Kinetics; D050505:Mutant Proteins / Q000737:chemistry; D017433:Protein Structure, Secondary; D012259:Ribonuclease, Pancreatic / Q000737:chemistry*
D005456:Fluorescent Dyes; D050505:Mutant Proteins; D012259:Ribonuclease, Pancreatic
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