Paracytophagy

Paracytophagy (from Ancient Greek para, meaning 'nearby', kytos, meaning 'cell', and phagy, meaning 'eating') is the cellular process whereby a cell engulfs a protrusion which extends from a neighboring cell. This protrusion may contain material which is actively transferred between the cells. The process of paracytophagy was first described as a crucial step during cell-to-cell spread of the intracellular bacterial pathogen Listeria monocytogenes, and is also commonly observed in Shigella flexneri. Paracytophagy allows these intracellular pathogens to spread directly from cell to cell, thus escaping immune detection and destruction. Studies of this process have contributed significantly to our understanding of the role of the actin cytoskeleton in eukaryotic cells.

Actin is one of the main cytoskeletal proteins in eukaryotic cells. The polymerization of actin filaments is responsible for the formation of pseudopods, filopodia and lamellipodia during cell motility. Cells actively build actin microfilaments that push the cell membrane towards the direction of advance.

Nucleation factors are enhancers of actin polymerization and contribute to the formation of the trimeric polymerization nucleus. This is a structure required to initiate the process of actin filament polymerization in a stable and efficient way. Nucleation factors such as WASP (Wiskott-Aldrich syndrome protein) help to form the seven-protein Arp2/3 nucleation complex, which resembles two actin monomers and therefore allows for easier formation of the polymerization nucleus. Arp2/3 is able to cap the trailing ("minus") end of the actin filament, allowing for faster polymerization at the "plus" end. It can also bind to the side of existing filaments to promote filament branching.

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