Actin Filament Architecture and Movements in Macrophage Cytoplasm

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Abstract
Actin filaments are the predominant structural elements in macrophage cortical cytoplasm. These fibres form a unique orthogonal network that fills all lamellae extended from the cell and which, in the cell body, bifurcates to form layers 0.2-0.5 micron thick on the cell top and bottom. Single short filaments, 0.1 micron in length, intersect in space in either T-shaped or X-shaped overlaps to form this ultrastructure. Network assembly and pseudopod extension occur when actin filaments within the network elongate. This filament growth is driven by a large storage pool of actin bound to the sequestering protein, profilin. Elongation is regulated by acumentin, gelsolin and possibly severin, proteins that bind to the end of the filaments, preventing the addition of actin monomers to the filaments. The cytosolic concentration of calcium controls whether filaments assemble or disassemble. Filaments can assemble when the filament ends are not blocked by gelsolin, a condition predicted to occur when the calcium concentration is less than 0.1 micron. Orthogonality results when actin filaments are cross-linked by molecules of actin-binding protein.