Regulation of receptor capping in mouse lymphoma T cells by Ca2+-activated myosin light chain kinase

Several characteristics of receptor capping in lymphocyte membranes suggest similarities with mechanisms underlying control of contraction in smooth muscle fibers. Both capping and contraction are Ca2+ dependent and require metabolic energy. Contractile proteins such as actin and myosin are associated with the cap, as is calmodulin, which mediates the Ca2+ dependence of smooth muscle contraction. Recent studies have shown that myosin light chain kinase (MLCK), which plays a central role in regulation of smooth muscle contraction, is also present in isolated lymphocyte membrane-cytoskeleton complexes. We have explored this analogy further, using mouse lymphoma T cells whose membranes were rendered permeable to small proteins by using a low-Ca2+ EGTA solution similar to that used to chemically skin smooth muscle cells. Permeabilized lymphocytes were then exposed to solutions containing various combinations of high or low Ca2+, ATP, or other nucleotides (5'-adenylyl imidodiphosphate, adenosine 5'-[gamma-thio]triphosphate, guanosine 5'-[gamma-thio]triphosphate, CTP, ITP, UTP, and GTP), calmodulin, Ca2+-insensitive MLCK (MLCK subunit that has been stripped of the Ca2+ binding site), and the catalytic subunit of cAMP-dependent protein kinase that phosphorylates (and thereby inactivates) MLCK. Capping of concanavalin A-labeled receptors in these various test solutions was scored. In all solutions the capping observed in permeable lymphoma cells correlated well with contraction previously observed in similarly treated skinned smooth muscle fibers, providing strong evidence for the involvement of myosin light chain phosphorylation in the regulation of receptor capping.