Colchicine induces the clustering of at least three different T-lymphoma surface antigens (T200, Thy-1, and gp 69/71) into a cap structure in the absence of any external ligand . In addition, colchicine induces the intracellular accumulation of actin and myosin directly beneath the surface cap structure . We have discovered that myosin molecules (both heavy and light chains) are closely associated with the plasma membrane of T-lymphoma cells. Most importantly, we have found that the 20,000-dalton light chain of lymphocyte myosin is both phosphorylated and preferentially accumulated in the plasma membrane of colchicine-induced capped cells. It is proposed that myosin light chain is directly involved in the activation of membrane-associated actomyosin required for the collection of surface proteins into a cap structure (analogous to muscle cell sliding filament contraction).Redistribution of lymphocyte surface receptors can be induced either with or without the addition of external ligands (15,18,23,29,30,37,41,(46)(47)(48) . During the process of receptor movement, surface components first form small clusters (patches), which then aggregate at one pole of the cell into socalled caps .In the case of ligand-dependent capping, the ligand and its receptors appear to be clustered by a cross-linking event at the cell surface that is highly dependent on the valency of the ligand (37). For example, divalent antibodies against surface immunoglobulin (Ig) are required for the induction of Ig capping; monovalent anti-Ig is unable to cause any redistribution of the Ig receptor (30) . In addition, capping induced by the lectin, concanavalin A (Con A), requires the tetrameric form of Con A (at the proper concentration) since the dimer form (e .g ., succinyl Con A) is not active in cap formation (15,18) . Information concerning ligand-independent capping is very limited . Recently, it has been found that hypertonic media, mitogens, trypsin, and cholinergic drugs are each able to induce surface receptors to form cap structures in the absence of any externally added ligand (29,(46)(47)(48). The molecular mechanisms underlying both ligand-dependent and ligand-independent capping are not yet understood .Previously, we have used a double immunofluorescence labeling technique to simultaneously determine the distribution of certain surface receptors and intracellular actin and myosin in human skin fibroblasts grown in monolayers (9) or in murine lymphoid cells grown in suspension cultures (7,10,11) . The THE JOURNAL of CELL BIOLOGY -VOLUME 91 DECEMBER 1981 889-894
