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Membranes are vital barriers by which cells control the flux of molecules and energy between their exterior and interior and also between their various intracellular compartments. While numerous transport systems exist for ions and small molecules, the cytosolic uptake of larger biological molecules and in particular antibody-targeted drugs, is a big challenge. Inducing leakage of the plasma membrane is unfavorable since the target cell specificity mediated by the antibody would likely be lost in this case. After binding and internalization, the antibody drug conjugates reach the endosomes. Thus, enforcing the endosomal escape of anti-tumor toxins without affecting the integrity of other cellular membranes is of paramount importance. Different strategies have been developed in the last decades to overcome endosomal accumulation and subsequent lysosomal degradation of targeted protein-based drugs. In this review we summarize the various efforts made to establish efficient techniques to disrupt the endosomal membrane barrier including the use of molecular ferries such as cell penetrating peptides or viral membrane fusion proteins, endosomal leakage inducing molecules such as saponins or monensin and physicochemical methods as represented by photochemical internalization.

Reovirus fusion-associated small transmembrane (FAST) proteins are the only known nonenveloped virus fusogens and are dedicated to inducing cell-to-cell, not virus–cell, membrane fusion. Numerous structural and functional attributes distinguish this novel family of viral fusogens from all enveloped virus membrane fusion proteins. Both families of viral fusogens play key roles in virus dissemination and pathogenicity, but employ different mechanisms to mediate membrane apposition and merger. However, convergence of these distinct families of viral membrane fusion proteins on common pathways needed for pore expansion and syncytium formation suggests syncytiogenesis represents a cellular response to the presence of cell–cell fusion pores. Together, FAST proteins and enveloped virus fusion proteins provide exceptional insights into the ubiquitous process of cell–cell membrane fusion and syncytium formation. [ABSTRACT FROM AUTHOR]

Membranes are vital barriers by which cells control the flux of molecules and energy between their exterior and interior and also between their various intracellular compartments. While numerous transport systems exist for ions and small molecules, the cytosolic uptake of larger biological molecules and in particular antibody-targeted drugs, is a big challenge. Inducing leakage of the plasma membrane is unfavorable since the target cell specificity mediated by the antibody would likely be lost in this case. After binding and internalization, the antibody drug conjugates reach the endosomes. Thus, enforcing the endosomal escape of anti-tumor toxins without affecting the integrity of other cellular membranes is of paramount importance. Different strategies have been developed in the last decades to overcome endosomal accumulation and subsequent lysosomal degradation of targeted protein-based drugs. In this review we summarize the various efforts made to establish efficient techniques to disrupt the endosomal membrane barrier including the use of molecular ferries such as cell penetrating peptides or viral membrane fusion proteins, endosomal leakage inducing molecules such as saponins or monensin and physicochemical methods as represented by photochemical internalization. [ABSTRACT FROM AUTHOR]

Abstract: The reovirus fusion-associated small transmembrane (FAST) proteins evolved to induce cell–cell, rather than virus–cell, membrane fusion. It is unclear whether the FAST protein fusion reaction proceeds in the same manner as the enveloped virus fusion proteins. We now show that fluorescence-based cell–cell and cell–RBC hemifusion assays are unsuited for detecting lipid mixing in the absence of content mixing during FAST protein-mediated membrane fusion. Furthermore, membrane curvature agents that inhibit hemifusion or promote pore formation mediated by influenza hemagglutinin had no effect on p14-induced cell–cell fusion, even under conditions of limiting p14 concentrations. Standard assays used to detect fusion intermediates induced by enveloped virus fusion proteins are therefore not applicable to the FAST proteins. These results suggest the possibility that the nature of the fusion intermediates or the mechanisms used to transit through the various stages of the fusion reaction may differ between these distinct classes of viral fusogens. [Copyright &y& Elsevier]

With no limiting membrane surrounding virions, nonenveloped viruses have no need for membrane fusion to gain access to intracellular replication compartments. Consequently, nonenveloped viruses do not encode membrane fusion proteins. The only exception to this dogma is the fusogenic reoviruses that encode fusion-associated small transmembrane (FAST) proteins that induce syncytium formation. FAST proteins are the smallest viral membrane fusion proteins and, unlike their enveloped virus counterparts, are nonstructural proteins that evolved specifically to induce cell-to-cell, not virus-cell, membrane fusion. This distinct evolutionary imperative is reflected in structural and functional features that distinguish this singular family of viral fusogens from all other protein fusogens. These rudimentary fusogens comprise specific combinations of different membrane effector motifs assembled into small, modular membrane fusogens. FAST proteins offer a minimalist model to better understand the ubiquitous process of protein-mediated membrane fusion and to reveal novel mechanisms of nonenveloped virus dissemination that contribute to virulence.