Johns Hopkins Medicine
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June 12, 2008
THE SHAPE-SHIFTING MECHANICS OF CELLS
Cell biologists at Johns Hopkins have discovered how tiny molecular
motors within cells work together with other structural players to
coordinate critical cell shape changes that accompany cell division.
The work appears in the April 8 issue of Current Biology.
"Cell division is a classic movement that all cells must do, so we
studied how several key proteins move during cell division to learn
more about the mechanics of cell shape change," says Douglas
Robinson, Ph.D., an associate professor of cell biology at the Johns
Hopkins School of Medicine.
Some cells like yeasts divide by pinching themselves into two, like
cinching a purse string. The purse string is made up of rope-like
rings of the actin protein within the cell. But Robinson's team
studies the social amoeba Dictyostelium, whose cells are similar to
human cells but are much easier to manipulate genetically. Using
high-power microscopes, the researchers discovered that rather than
forming purse string-like rings, actin in Dictyostelium cells organizes into
mesh-like structures in the region where the cell will divide.
"This was a surprise," says Robinson, "because the textbooks teach us
that the myosin-II motor slides along and pulls these actin purse
strings together to pinch a dividing cell, but we couldn't imagine a
way that the same mechanics could work with short segments of
actin in a weblike structure."
To figure out how the myosin-II motor contracts an actin mesh during
cell division, the research team looked at cells containing, instead
of normal myosin-II, an altered form of myosin-II that works 10 times
slower than normal, cells lacking myosin-II altogether and normal
cells. They found that cells lacking myosin-II divided the fastest,
which suggested that myosin-II isn't the only thing controlling cell
shape change during division.
"Cells are like liquid-filled balloons with a consistency thicker
than water and closer to honey or Jell-O," says Robinson, "so we
decided to look at the deformability of each of these different
cells." To do this, they used tiny capillary pipettes that "like
straws sucking on the surface" can measure the hardness or softness
of the cell.
The researchers found that cells lacking myosin-II are more
deformable than normal cells. Normal cells in the process of
dividing, however, were stiffer near the division site than elsewhere
in the cell.
The team then examined the deformability of cells containing or
lacking so-called actin crosslinkers, proteins that bind to both the
actin meshwork and myosin-II. They found that cells lacking the
crosslinkers were more deformable, suggesting that myosin-II works
with crosslinkers to increase tension and, with that, increase
elasticity to enable the cell to change shape where it's dividing.
"Structure itself is not the critical part," says Robinson. Instead,
structure may be the effect rather than the cause of the shape change.
The research was funded by the National Institutes of Health and
National Science Foundation.
Authors on the paper are Elizabeth M. Reichl, Michael Delannoy, Janet
C. Effler, Kristine D. Girard, Srikanth Divi, Pablo A. Iglesias, Scot
C. Kuo, and Douglas N. Robinson, all of Johns Hopkins, and Mary K.
Morphew of the University of Colorado.