Evidence for contractile protein translocation in macrophage spreading, phagocytosis, and phagolysosome formation.

Abstract

Rabbit lung macrophages were allowed to spread on nylon wool fibers and then subjected the adherent cells to shear. This procedure caused the selective release of .beta.-glucuronidase into the extracellular medium and yielded 2 fractions, cell bodies and isolated pseudopod blebs resembling podosomes, which are plasmalemma-bounded sacs of cortical cytoplasm. Cytoplasmic extracts of the cell bodies eluted from nylon fibers contained 2/3 less actin-binding protein and myosin, and .apprx. 20% less actin than extracts of cells sheared in the absence of nylon wool fibers. Nearly all of the actin and 2/3 of the other 2 proteins were accounted for in podosomes. The alterations in protein composition correlated with assays of myosin-associated EDTA-activated ATPase activity, and with a diminution in the capacity of extracts of nylon wool fiber-treated cell bodies to gel, a property dependent on the interaction between actin-binding protein and F-actin. The capacity of the remaining actin in cell bodies to polymerize did not change. Actin-binding protein and myosin are apparently concentrated in the cell cortex and particularly in pseudopodia where prominent gelation and syneresis of actin occur. Actin in the regions from which actin-binding protein and myosin are displaced disaggregates without depolymerizing, permitting lysosomes to gain access to the plasmalemma. Translocation of contractile proteins could account for the concomitant differences in organelle exclusion that characterize phagocytosis.