proteins bound to actin, called troponin and tropomyosin. When calcium is low, troponin and tropomyosin interfere with actin and myosin interaction. There are three polypeptides associated with the troponin molecule. The first is troponin I, which is inhibitory; the second is troponin T, which binds to tropomyosin; the third is troponin C, which binds to the calcium molecule. It takes the binding of calcium to troponin C specifically to allow the troponin and myosin to interact. In non-muscle cells, actin is still contractile and there are filaments of myosin II associated with this. Tropomyosin is also involved but there are no striations, M lines, or Z discs involved. There are adhesion belts that help to alter the shape of the non-muscle cell. This is what happens in the cytokinesis process so that one cell can pinch off into two cells. Calcium is not involved but there is phosphorylation or the addition of a phosphate group to myosin that assembles myosin and allows it to contract. There is an enzyme called myosin light-chain kinase that performs this reaction. The kinase itself is regulated by calmodulin, which binds to calcium in the cell so that phosphorylation can occur.
CELL MIGRATION Cell locomotion in non-muscle cells is also called cell crawling. It is important in embryonic development, the immune response, and the metastasis of cancerous cells. The actin cytoskeleton is very important to this cell crawling process. Phagocytosis is another type of cell crawling that is mediated by the actin filaments. Cell migration happens because of external physical, mechanical, or chemical triggers. It is necessary in all forms of life, including humans. The gastrulation of embryos takes place because of cell migration. Tissue repair and renewal involves cell migration. Movement of a cell is a cyclic process. There are filopodial or lamellipodial protrusions, adhesion of the cell to the matrix, and the pushing of the cell over the adhesions so that the cell moves. The first part of the process is the polarization of the cell. There are GTPase enzymes that help to determine what is the front of the cell and what is the back of the cell. Microtubules grow at the leading edge and anchor the cell. This facilitates the pulling of the rest of the cell across the adhesions. 183
