Although it is widely known that many cells are polarized, the mechanism by which this polarity is manifested is not very well understood. In research recently published in Developmental Cell, University researchers are beginning to elucidate these mechanisms.
Felipe Santiago, grad, in the department of molecular biology and genetics, researched this polarity through the cell division of Saccharomyces cerevisiae (budding yeast). According to Santiago, budding yeast “is a very nice model because it grows in a very asymmetric way.” Instead of growing in all directions like human cells and then dividing down the middle, budding yeast grow the daughter cell out of one end of the parent cell. The asymmetry of the common process of cell division allows scientists to easily study cell growth and polarity.
Most cells have a cytoskeleton that acts as the cell’s scaffolding, giving the cell shape and structure. The cytoskeleton also acts as a highway system for certain proteins called motor proteins that transport cellular materials in membranes called vesicles around the cell. Before the budding yeast cell begins to form the daughter bud, the cytoskeleton is organized such that they point to the bud. This forms the roads that allow the necessary materials for growth to be transported to the bud.
Santiago’s adviser, Prof. Anthony Bretscher, molecular biology and genetics, asserted, “the most interesting thing in biology is how cells work … having the right proteins on the right surface at the right amount at the right time.”
Santiago discovered that two types of molecules –– Rab GTPases, a family of proteins, and phosphoinositides (PIs), a family of lipids –– must associate with Myosin V, a motor protein, before it can transport the proteins necessary for the asymmetrical cell growth to the bud. Without either molecule, the activity of the Myosin V and cell growth is severely inhibited.
This system is called “coincidence detection” and it allows for much greater specificity and control over the regulation of the growth proteins and Myosin V. The two molecules correspond to two different requirements that need to be fulfilled before the Myosin V should transport the growth proteins, but the exact circumstances of when the molecules bind to the Myosin V are not yet known.
The research involved genetic, microscopy and biochemical methods. In the genetic component, budding yeast were genetically altered so that one of the two proteins was missing or the motor protein’s ability to interact with proteins was altered. In all three cases, cell growth was inhibited and could only be saved by increasing the concentration of the other factors.
To visualize the molecules’ choreograhy, proteins and lipids were stained so the researchers could observe their locations in the cell. It was found that the Rab GTPases and PIs localize in the same places in the cell, suggesting that they worked in tandem in some form.
Finally, in the biochemical component, the two molecules were purified and isolated with the Myosin V in vitro. Santiago found that the Rab GTPases were able to bind to the Myosin V while the PIs were not able to, suggesting a yet discovered third protein that enables the PIs to bind to the Myosin V.
Santiago explained the significance of his work. “There are two different big things. One of them is the identification of the Rab proteins and the PIs as the signals to recruit the Myosin. The other one is that usually people think that in the cell, the proteins are the ones that are involved in carrying out the functions … but in here, what we are seeing is that lipids are not only parts of the membranes, but that they can also be signaling molecules on their own.”
As Bretscher stated, “After 20 years, only now are we beginning to understand the process in more elaborate detail.”