Sep. 5, 2000 ITHACA, N.Y. -- Cornell University biologists have shown how tiny molecular motors carrying target proteins help orient the spindle-like apparatus that transfers genetic material from the nucleus of a mother cell to the daughter. The research explains an essential mechanism in the birth of a new cell, and how failures of molecular motors can have dire consequences for new cell formation.
The new model for mitotic mechanics, as reported in the latest issue of the journal Nature (Aug. 31, 2000, Vol. 406, No. 6799, pp. 1013-15), was worked out in budding yeast cells. But the study is expected to prompt further research into whether similar processes occur in the formation of cells of all higher organisms, including humans.
"The process of properly orienting the spindle with the axis of cell division has to be incredibly accurate, otherwise cells run the risk of not transferring their genetic material into newly formed daughter cells," explains Anthony Bretscher, professor of molecular biology and genetics at Cornell and one of four authors of the report ("Myosin V orients the mitotic spindle in yeast"). "We found the protein Myo2p works as a molecular motor by walking along oriented actin cables [protein fibers] while carrying another protein, Kar9p, to the yeast bud. This is the first time anyone could draw a molecular mechanism for spindle orientation in yeast. The big question is: Does this also happen in human cells?"
Molecular motors are tiny specialized structures that can move and perform tasks within cells, fueled by cellular energy in the form of ATP (adenosine triphosphate). Mitosis is the essential process in the formation of new cells by which the mother cell's nucleus divides to provide a duplicate set of genetic instructions to the developing daughter cell in the form of chromosomes.
In yeast, mitosis depends upon two systems of fibers within the cell. Actin fibers form tracks that lead from the mother cell and into the daughter cell, or bud, in the case of yeast. A second set of fibers, called microtubules, form a spindle (also called the mitotic spindle) that ensnares and then separates the chromosomes. During mitosis, this spindle must elongate so that one of its ends pushes into the daughter cell, dragging with it one complete set of chromosomes. Mitosis requires proper orientation of the spindle toward the new, developing cell; otherwise the duplicated chromosomes remain in the mother cell and development of the daughter cell fails for lack of genetic instructions.
This transfer of genetic material is absolutely essential for all organisms because without genetic instructions, new cells cannot develop. Even minor problems in mitosis can cause serious defects such as the development of cancerous cells.
Cell biologists have recognized in yeast that actin cables somehow guide the spindle of microtubules into the bud during orientation of the spindle. However, how these two sets of fibers were linked was unclear. Recent studies at Princeton University identified the protein Kar9p as part of the link that draws the microtubules of the spindle into the correct orientation. Kar9p, which is found in the bud, is able to grab hold of microtubule ends.
"But what we didn't know was how Kar9p gets from the mother cell into the bud," said Bretscher, crediting Cornell graduate student Hongwei Yin with experiments that completed the mitotic mechanics picture. Other Cornell authors of the Nature report are Tim C. Huffaker, an associate professor of molecular biology and genetics, and David Pruyne, a post-doctoral researcher in that field.
Yin found the link by examining the role of the molecular motor protein Myo2p. The Myo2p motor was known to walk along the actin cables, leading from the mother cell into the bud. Using biochemical and genetic assays, Yin found that Kar9p and Myo2p are able to bind to each other. Furthermore, Yin found that Myo2p is required to guide Kar9p into the bud. Yin used yeast with partially defective forms of the Myo2p molecular motor protein. These defective motors behaved normally under most conditions, but failed above 35 degrees Centigrade (95 degrees Fahrenheit).
The results, she found, were defects in mitosis in budding yeast cells: The Kar9 protein was not carried into the bud, the mitotic spindle was no longer guided by actin cables, and chromosomes were no longer efficiently transferred into the daughter. In contrast, yeast with fully functional Myo2p motors efficiently shuttled Kar9p along actin cables into the bud, allowing Kar9p to pull the tips of microtubules into the bud and properly orient the yeast cells' mitotic spindles, as revealed by immunofluorescence microscopy. Yin's work is the first to successfully demonstrate the role of a molecular motor guiding the spindle during mitosis -- a small step for a cell's nucleus and a giant leap for cellular life. The mitosis studies at Cornell were funded by grants from the National Institutes of Health.
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