New Frontiers in Cell Biology
Gavin King, an associate professor of physics, is trying to understand one of the most basic concepts in cell biology: secretion, or the mechanism by which proteins can pass through a cell membrane to get from one compartment of a cell to another. Membrane proteins are the “gatekeepers” that allow information and molecules to pass into and out of a cell.
“It turns out to be a really fundamental process in biology, because in order to regulate itself in the context of its environment, a cell has to secrete or move proteins across membranes,” King says.
In order to study this phenomenon, King and his colleagues used atomic force microscopy (AFM),a method that was invented in 1986 to study solid-state surfaces, but has recently been applied in biology. King describes the tool as a nanoscale robotic arm with a very sharp tip, where that tip oscillates very gently up and down over a sample, much like how a blind person might read Braille. He says the AFM can basically study anything one can place on a surface.
In this case, King is looking at one particular protein that resides in the cell of E-coli. By binding a protein (SecA) or an enzyme to a surface, King can image it with the AFM and explore various reactions when the enzyme is exposed to nucleotides (a nucleotide consists of a base plus a molecule of sugar and one (or more) of phosphoric acid, such as adenosine triphosphate or ATP, the energy currency of the cell).
“We can’t control when ATP binds to the protein molecule, but we can image continuously—keep our eyes on this one molecule—so we add ATP, and then we watch what happens,” King says. “It’s like making a movie of a single molecule doing its biological work.”
For King, the big picture question is trying to figure out how the secretion process works. He describes the process as reverse engineering the cell.
“E-coli cells are very robust, efficient machines,” King says. “Those of us with a physical science background like to understand how machines work, so we smash open the machine,” he says. “We have this big cell—how does it work? We smash it open, take each of the components out, and study them individually, and that’s what we are doing here with SecA. We’re putting SecA under our microscope and asking how does its shape and behavior change as a function of these nucleotides or these different chemicals we are subjecting them to in an effort to gain fundamental knowledge about how the secretion process works.”
There are competing models of how the secretion process works, and King’s group is trying to determine which model is correct or if there is another model that might explain the process. His group is the first to apply atomic force microscopy to study the components of the general secretory system.
Cartoon of the E. coli general secretory system. The secreted protein (i.e., the precursor protein) is drawn in red.
One member of King’s team is Professor Emerita of Biochemistry Linda Randall, who drew the illustration in this article. “I’m good at tools and equipment, and she’s good at biology. When you put us together some interesting science can occur, and this is a good example,” King says. What is missing in biology is the ability to predict behaviors from a set of first principles —how a cell would react when introduced to a certain chemical or how the cell would behave if a scientist made a mutation on a protein. He says it’s nearly impossible to make predictions because scientists don’t know the details for a number of biological processes such as secretion.
Understanding how this process works could lead to the development of more effective pharmaceuticals, for example, although King says this type of basic research is the foundation of a research-based, land-grant institution like MU.
“When you are talking about the basic understanding of a process like this, it’s almost impossible to anticipate what that knowledge is going to produce,” King says.