You are what you eat and drink. Steak can sit in your stomach or orange juice wind through your intestines, but they only
become part of your body once they''re taken up by your cells. First, foods must be reduced to a soup of proteins, fats, sugars, and so on. But even then, getting these materials into a cell isn''t as simple as sticking them in your mouth. For one, there''s the
membrane enclosing a cell. Simply puncturing a hole in the membrane would spill the cell''s contents, harming or killing the cell.
Instead, all eukaryotesorganisms whose cells have nucleiuse a carefully orchestrated process called endocytosis to bring materials into their cells. Eukaryotic cells first form cavities in their cell membrane that surround nearby particles or fluid. These pockets seal shut and bud off into the cell to form small membrane-bound sacs called vesicles.
When taking in fluids, eukaryotic cells use two distinct mechanismsto take tiny sips or huge gulps. With one process, called pinocytosis, cells continually form small pockets in the cell membrane that enclose small droplets of fluid in vesicles called pinosomes. These newly formed vesicles, called early endosomes, bud off from the membrane and fuse with other early endosomes. In one form of pinocytosis, the vesicles are encaged by a protein called clathrin that tightly constrains their size. These carriers incorporate membrane constituents (for example, growth factors) with very high selectivity. In
macropinocytosis, on the other hand, large ruffles in the membrane engulf mass quantities of fluid in vesicles known as macropinosomes.
Beyond taking in nutrients, these processes are essential to the function of many organsfrom the brain, where nerve cells receive other cells'' chemical signals by pinocytosis, to the kidney, where cells use macropinocytosis to take in waste fluids for processing. Macropinocytosis is also relevant to cancer cells; it has long been known that oncogenes dramatically induce this endocytic process, affecting the signaling status of these cells. But compared with other types of endocytosis, molecular biologists know surprisingly little of the mechanisms behind macropinocytosis. They do know that the Rab5 proteinan enzyme that coordinates a complex network of other proteins, called effectorsis crucial for both pinocytosis and macropinocytosis.
Now, as reported in this issue of PLoS Biology, Marino Zerial and colleagues have found a new protein, which they named Rabankyrin-5, that forms a further link between these two mechanisms for fluid uptake. The protein is necessary for macropinocytosis, and its levels control the rate of this process. In addition, Rabankyrin-5 helps regulate endosome trafficking and coordinates this mechanism with macropinocytosis.
In two commonly used human and mouse cell lines, the researchers found the protein Rabankyrin-5 along with Rab5 on both types of pinosomes, early endosomes and macropinosomes. The early endosomes usually fuse with one another inside the cell, but when the researchers blocked Rabankyrin-5 activity, this fusion fell sharply. Suppressing Rabankyrin-5 activity also stifled macropinocytosis; overexpressing the effector, on the other hand, sent macropinocytosis into overdrive.
The researchers also looked at endocytosis in mouse kidney and canine kidney cell lines. Inside the kidney, fluid-carrying ducts are lined with epithelial cells that take up liquids through their exposed surface. The researchers found Rabankyrin-5 predominately on vesicles at this surface, and as in the other experiments, overexpression of the protein promoted macropinocytosis. Together, these findings suggest Rabankyrin-5 plays a role in regulating this form of fluid uptake and plays a role in kidney function. The discovery of Rabankyrin-5 involvement in macropinocytosis also has implications for other physiological and pathological mechanisms such as the immune system response, defense against pathoge