Cell-penetrating Peptide - Mechanisms of Membrane Translocation - Direct Penetration

Direct Penetration

The majority of early research suggested that the translocation of polycationic CPPs across biological membranes occurred via an energy-independent cellular process. It was believed that translocation could progress at 4oC and most likely involved a direct electrostatic interaction with negatively charged phospholipids. Researchers proposed several models in attempts to elucidate the biophysical mechanism of this energy-independent process. Although CPPs promote direct effects on the biophysical properties of pure membrane systems, the identification of fixation artifacts when using fluorescent labeled probe CPPs caused a reevaluation of CPP-import mechanisms. These studies promoted endocytosis as the translocation pathway. An example of direct penetration has been proposed for Tat. The first step in this proposed model is an interaction with the unfolded fusion protein (TaT) and the membrane through electrostatic interactions, which disrupt the membrane enough to allow the fusion protein to cross the membrane. After internalization, the fusion protein refolds due the chaperon system. This mechanism was not agreed upon, and other mechanisms involving clathrin-dependent endocytosis have been suggested.
Recently, a detailed model for direct translocation across the plasma membrane has been proposed. This mechanism involves strong interactions between cell-penetrating peptides and the phosphate groups on both sides of the lipid bilayer, the insertion of charged side-chains that nucleate the formation of a transient pore, followed by the translocation of cell-penetrating peptides by diffusing on the pore surface. This mechanism explains how key ingredients, such as the cooperativity among the peptides, the large positive charge, and specifically the guanidinium groups, contribute to the uptake. The proposed mechanism also illustrates the importance of membrane fluctuations. Indeed, mechanisms that involve large fluctuations of the membrane structure, such as transient pores and the insertion of charged amino acid side-chains, may be common and perhaps central to the functions of many membrane protein functions. This model contains several controversial features, maybe the most striking one is the formation of transient pores that facilitate the diffusion of the peptides across either the plasma membrane or the endosomal vesicles towards the cytosol. Recent experimental data has validated this key ingredient of the model showing that cell-penetrating peptides indeed form transient pores on lipid bilayers and on live cells.

Read more about this topic:  Cell-penetrating Peptide, Mechanisms of Membrane Translocation

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