Cell membranes contain millions of embedded proteins that control ion movements into and out of the cell. This ion flow underlies such vital functions as electrical signaling in nerve, heart, and muscle cells; cell volume regulation; secretion of hormones and neurotransmitters; fertilization; and kidney function. Dr. Gadsby’s research focuses on how ion transport proteins work.
Two principal classes of proteins regulate ion movement across membranes: pumps and channels. Although both move ions through the cell’s otherwise impenetrable phospholipid bilayer, they play distinct roles. Channels allow ions to flow rapidly down their electrochemical gradients, while pumps move ions relatively slowly, thermodynamically uphill, thereby building up those gradients. Pumps and channels have therefore traditionally been viewed as very different entities. Dr. Gadsby’s work, however, suggests they are far more closely related than generally assumed.
The Gadsby lab is using position-specific mutagenesis, combined with structural modeling and biochemical and electrical measurements, to examine the mechanisms of two biomedically important ion transport proteins. One, the Na+/K+-adenosine triphosphatase, is a pump crucial to animal cell life, and the other, CFTR (cystic fibrosis transmembrane conductance regulator), is a Cl⁻ ion channel. Mutations in the CFTR gene are responsible for cystic fibrosis. Mutations in the Na+/K+ pumps of brain neurons have been found responsible for childhood neurological disorders.
Research from the Gadsby lab suggests that, whereas an ion channel can be viewed as a transmembrane ion pathway controlled by a gate at one end, an ion pump can be viewed as a modified ion channel governed by gates at both ends. A pump’s gates must be tightly coupled so that both are never open simultaneously. From this perspective, the conformational changes that open and close the ion pathway gates in the two kinds of transport need not be all that different. Indeed, in both the Na+/K+ pump and CFTR Cl⁻ channel, these conformational changes are driven by binding and hydrolysis of adenosine triphosphate (ATP).
In work on CFTR, lab members use the sensitive patch-clamp recording technique to analyze the timing of the opening and closing of individual wild-type and mutant CFTR channels as they are exposed to ATP and/or nucleotide analogues. Based on their findings, the Gadsby lab has detailed the accepted model for how opening and closing of the CFTR ion channel are regulated, furthering understanding of the origins of cystic fibrosis.
Work on the Na+/K+ pump has used a marine toxin that interferes with the strict coupling between the pump’s two gates. When the toxin is bound to the pump, the two gates are occasionally both open, and the pump is transformed into an ion channel. Dr. Gadsby is exploiting this action to probe the pathway through the Na+/K+ pump as well as the nature and control of its gates.
Most recently, the Gadsby lab has found that during the normal Na+/K+ transport cycle, a certain conformation of the Na+/K+ pump can be hijacked by extracellular protons to access the cell interior. The probability of proton entry through any given Na+/K+ pump depends on the extracellular proton concentration. The Na+/K+ pump is thus a hybrid transporter, a protein with two distinct functions.