Related Experiment Videos
Myosin-I in mammalian liver
1Department of Biochemistry, Emory University School of Medicine, Atlanta, Georgia 30322.
Insights
Researchers identified two novel myosin-I polypeptides in rat liver, crucial for cell motility. These proteins bind actin and calmodulin, suggesting multiple myosin-I forms exist in higher organisms and within single cell types.
Area of Science:
- Biochemistry
- Cell Biology
- Molecular Biology
Background:
- Myosin-I is a class of 110-kDa proteins involved in cellular motility processes like migration and phagocytosis.
- While found in various higher eukaryotic tissues, the specific forms and functions of myosin-I in mammalian liver were previously unclear.
Purpose of the Study:
- To purify and characterize myosin-I polypeptides from rat liver.
- To investigate the functional characteristics and relationships of these liver myosin-I forms.
Main Methods:
- Protein purification using ATP solubilization and gel filtration.
- Biochemical assays including calmodulin binding, F-actin binding, and ATPase activity measurements.
- Immunological characterization using polyclonal antisera and proteolysis studies.
Main Results:
- Two distinct rat liver polypeptides (110-kDa and 130-kDa) with myosin-I characteristics were purified.
- Both polypeptides bind F-actin and calmodulin, exhibit actin-activated Mg(2+)-ATPase activity, and crosslink actin filaments.
- Immunological data and proteolysis indicate the polypeptides are distinct but related, with specific antibodies recognizing each.
Conclusions:
- Multiple myosin-I molecules are present in higher organisms, potentially coexisting within a single cell type.
- The purified rat liver polypeptides represent novel forms of myosin-I with functional similarities to brush border myosin-I.
Abstract:
Myosin-I refers to a class of proteins with a molecular weight of approximately 110-kDa, which have characteristics of conventional myosin but are unable to form filaments. Previous studies have implicated myosin-I in motile cellular processes including cell migration and phagocytosis. Although the first example of myosin-I in higher eukaryotes was the intestinal 110K-calmodulin complex, which forms in microvilli the lateral links connecting the core bundle of actin filaments to the membrane, myosin-I has now been shown to be a component of rat kidney and to be present in bovine adrenal gland and brain. We have now purified and characterized two polypeptides from rat liver which have several characteristics of the intestinal 110K-calmodulin complex. Both liver polypeptides are solubilized with ATP and co-elute on gel filtration with calmodulin. The polypeptides, of 110-kDa and 130-kDa, bind calmodulin in 1 mM EGTA. Both polypeptides bind to F-actin in an ATP reversible fashion, and crosslink actin filaments. The purified polypeptides possess an actin-activated Mg(2+)-ATPase activity typical of brush border myosin-I. A polyclonal antiserum directed against the chicken intestinal 110-kDa polypeptide recognizes both rat liver polypeptides, whereas another serum recognizes the 130-kDa but not the 110-kDa rat liver polypeptide. Controlled proteolysis of the purified polypeptides with alpha-chymotrypsin indicates that the two polypeptides are distinct but related. Immunofluorescence microscopy on isolated hepatocytes shows distribution of myosin-I to be vesicular, distributed throughout the cytoplasm, but more concentrated near the nucleus. These data contribute new evidence by several functional criteria that multiple myosin-I molecules are present in higher organisms and may coexist in a single cell type.