SUBCELLULAR-LOCALIZATION OF MOESIN IN DYNAMIC FILOPODIA, RETRACTION FIBERS, AND OTHER STRUCTURES INVOLVED IN SUBSTRATE EXPLORATION, ATTACHMENT, AND CELL-CELL CONTACTS

Moesin, a member of the talin-4.1 superfamily, is a linking protein of the submembraneous cytoskeleton. It is expressed in variable amounts in cells of different phenotypes such as macrophages, lymphocytes, fib roblastic, endothelial, epithelial, and neuronal cell lines. In this r eport we show that moesin is not randomly distributed throughout the c ortical cytoskeleton, but rather that it is concentrated in specialize d microdomains. It is localized in the intracellular core of microexte nsions known as filopodia, microvilli, microspikes, and retraction fib ers. This subcellular distribution follows closely the dynamic changes in cell shape that take place when cells attach, spread, and move spo ntaneously or in response to extracellular signals. This suggests a si milar function for moesin in diverse cell types related to the dynamic restructuring of domains of the plasma membrane and underlying membra ne skeleton. Support for this comes from studies on PC-12 cells, which respond to NGF by extending neurites and moesin is redistributed from a diffuse localization to growth cone filopodia. In fibroblastic (NIH 3T3) or macrophage (RAW264.7) cell lines, moesin is found in filopodia appearing at random on the cell surface soon after the cells are plac ed in culture, begin to attach, and spread, In polarized epithelial ce lls (LLC-PK1), moesin is associated with peripheral filopodia and apic al microvilli. The cellular microextensions containing moesin are devo id of microtubules, focal contact proteins such as vinculin, and corti cal cytoskeletal elements such as protein 4.1, but they do contain var ying amounts of actin microfilaments. This localization of moesin in m icroextensions is not influenced by cytochalasin B. Treatment of cells with phorbolester (PMA) causes sapid cell spreading, disappearance of filopodia and retraction fibers, and moesin does not accumulate in th e actin-rich lamellae that form at the cellular edges. After removal o f PMA, cells retract and moesin again becomes concentrated in filopodi a and retraction fibers. These studies support the hypothesis that fil opodia, retraction fibers, and other microextensions of the plasma mem brane are unique cellular microdomains with characteristic submembrane ous components. Moesin could be involved in the dynamic restructuring of such microdomains by regulating binding interactions between the pl asma membrane and the actin cytoskeleton. (C) 1995 Academic Press, Inc .