During Multicellular Migration, Myosin II Serves a Structural Role Independent of its Motor Function

We have shown previously that cells lacking myosin II are impaired in multicellular motility. We now extend these results by determining whether myosin contractile function is necessary for normal multicellular motility and shape control. Myosin from mutants lacking the essential (mlcE−) myosin light chain retains the ability to form bipolar filaments that bind actin, but shows no measurable in vitro or in vivo contractile function. The contractile function is necessary for cell shape control since mlcE− cells, like myosin heavy-chain null mutants (mhcA−), were defective in their ability to control their three-dimensional shape. When mixed with wild-type cells in chimeric aggregation streams, the mlcE− cells were able to move normally, unlike mhcA− cells which accumulated at the edges of the stream and became distorted by their interactions with wild-type cells. When mhcA− cells were mixed with mlcE− streams, the mhcA− cells were excluded. The normal behavior of the mlcE− cells in this assay suggests that myosin II, in the absence of motor function, is sufficient to allow movement in this constrained, multicellular environment. We hypothesize that myosin II is a major contributor to cortical integrity even in the absence of contractile function.