PROTRUSIVE ACTIVITY, CYTOPLASMIC COMPARTMENTALIZATION, AND RESTRICTION RINGS IN LOCOMOTING BLEBBING WALKER CARCINOSARCOMA CELLS ARE RELATEDTO DETACHMENT OF CORTICAL ACTIN FROM THE PLASMA-MEMBRANE

The dynamic events at the front of locomoting blebbing Walker carcinos arcoma cells [Keller and Bebie, Cell Motil. Cytoskeleton 33:241-251, 1 996] are interpreted on the basis of an analysis of the actin cytoskel eton and its relationship to the plasma membrane in fixed cells using a novel double-stainin procedure. The data show that blebs are formed where cortical actin is locally depolymerized and/or by detachment of the plasma membrane from more or less intact cortical actin layers. Di ssociation between the cortical actin layer and the plasma membrane, w hich is stimulated by microtubule disassembly, is achieved by forward movement of the plasma membrane, rather than by retraction of the acti n layer. Therefore, the detached actin layers form a boundary between the newly forming protrusions and the rest of the cell. They can be as sociated with ''constriction rings,'' which we have termed ''restricti on rings.'' Detached actin layers can impede entry of organelles and t he nucleus into the protrusions and thereby compartmentalize the cytop lasm. Later, detached cortical actin layers depolymerize, allowing for relaxation of the restriction rings and for forward movement of cytop lasmic organelles and the nucleus. Actin may repolymerize along the de tached plasma membrane allowing for a new cycle to occur. Estimates in dicate that the actin polymerization/depolymerization cycles may be la rgely confined to the front of blebbing cells. The findings suggest th at the dynamic events at the front of blebbing metazoan cells are simi lar to those previously found in Amoeba proteus [Grebecki, Protoplasma , 154:98-111, 1990] but different from those found in lamellipodia. Ce ll Motil. Cytoskeleton 41:181-193, 1998. (C) 1998 Wiley-Liss, Inc.