Roderick Mackinnon, MD, Honored by 2003 Nobel Prize in Chemistry for Work on Elucidating the Structure of Ion Channels

SPECIAL COMMENTARY J Am Soc Nephrol 15: 1096–1097, 2004

Roderick MacKinnon, MD, Honored by 2003 Nobel Prize in Chemistry for Work on Elucidating the Structure of Ion Channels

STEVEN C. HEBERT and GERHARD GIEBISCH Department of Cellular and Molecular Physiology, Yale University School of Medicine, New Haven, Connecticut.

This past year, Rod Mackinnon shared the Nobel Prize in Prize in Chemistry, Rod MacKinnon also received the Lasker Chemistry for his groundbreaking discoveries in defining the Award in 1999, the Rosenstiel Award in 2000, and the Gaird- structures of potassium and chloride channels. This work sheds ner Award in 2001. In 2000, he was also honored by the new light on the mechanisms by which channels determine Hodgkin-Huxley-Katz Prize of the British Physiologic Society which ion to permit passage (called selectivity) and open or and was elected to the US National Academy of Sciences. close (gate) in response to a variety of extracellular and intra- After receiving a BA in Biochemistry from Brandeis in 1978 cellular molecules or ligands (e.g., voltage). Dr. MacKinnon is and the MD from Tufts, he returned to Brandeis for postdoc- the third recipient in a Nobel trilogy that has defined the toral studies in the laboratory of Christopher Miller, PhD, modern era of ion channels. In 1963, Sir John Carew Eccles, where he began his scientific journey on ion channel function. Sir Alan Lloyd Hodgkin, and Sir Andrew Fielding Huxley He joined the faculty at Harvard Medical School in 1999, shared the Nobel Prize in Medicine for their discoveries of the where he used electrophysiological, biochemical, and molecu- ionic mechanisms underlying the action potential—the idea lar biologic tools to identify regions in Kϩ channels that that ion fluxes across a cell membrane generate electrical control ion selectivity and gating. Realizing that an in-depth impulses. In 1991, Erwin Neher and Bert Sakmann shared the understanding of these cardinal channel properties would re- Nobel Prize in Medicine for their work providing the means for quire visualization of the Kϩ channel protein structure, he directly visualizing ion currents through single channels (i.e., began his pursuit of this goal after joining the faculty at the invention of patch clamping). This work demonstrated that Rockefeller University. This was a lofty but difficult goal, as pores through biologic membranes provide a pathway for the very few membrane proteins, in contrast to soluble proteins, rapid flux (up to millions of ions per second) of ions that had been crystallized. Many thought he was embarking on a underlie the electrical properties of cells. This technology fool’s errand. He succeeded! subsequently allowed identification and classification of the Over the past 40 years, we have come to understand the various types of channels based on the ion that permeates the fundamental importance of ion channels in cell biology (in- ϩ ϩ Ϫ ϩ pore (e.g., ionic selectivity for K ,Na ,Cl ,orCa2 ), the cluding renal physiology and pathophysiology). Ion channels electrical conductance, and the mechanism for opening and set the cell membrane potential, generate electrical signals in closing of the pores (channel gating). The third component of excitable cells, regulate cell volume and cell movement, as the trilogy is the work of Rod MacKinnon and collaborators, well as mediate net transport of certain ions in renal epithelia which provides the first x-ray crystallographic snapshots of (e.g.,Kϩ secretion in distal nephron segments). Ion channels channels. These latter seminal discoveries elucidated the struc- are also common targets for pharmaceutical agents. Moreover, ϩ Ϫ tural basis for the K and Cl channel selectivity and gating, mutations in ion channel genes have been identified in a that is, the fundamental properties that make an ion channel. number of inherited diseases–the ROMK Kϩ channel gene Roderick MacKinnon is the John D. Rockefeller Jr. Profes- (KCNJ1) and the ClϪ channel gene (CLC-KB) in Bartter syn- sor and Head of the Laboratory of Molecular Neurobiology and drome and the Naϩ channel gene (ENaC) in Liddle syndrome, Biophysics at Rockefeller University and an Investigator of the to name only a few. Howard Hughes Medical Institute. In addition to the Nobel Rod MacKinnon’s work has unveiled the beauty and econ- omy of nature in defining the structural basis of ion selectivity and gating by Kϩ channels. The movement of ions through Correspondence to Dr. Gerhard Giebisch, Department of Cellular and channels is governed by conformational changes resulting in Molecular Physiology, Yale University School of Medicine, 333 Cedar St., New Haven, CT. Phone: 203-785-4076; Fax: 203-785-4951; E-mail: channel opening or closing (i.e., gating) that respond to mem- [email protected] brane voltage and/or a variety of extracellular and intracellular 1046-6673/1504-1096 molecules or ligands. The means by which ion channels are Journal of the American Society of Nephrology able, on the one hand, to discriminate between anions and Copyright © 2004 by the American Society of Nephrology cations (and even between cations like Kϩ and Naϩ) and, on DOI: 10.1097/01.ASN.0000118342.72706.69 the other, to allow rapid passage of the ions has been a J Am Soc Nephrol 15: 1096–1097, 2004 Roderick MacKinnon, MD 1097

electrical signals in nerve and muscle. In 2001, MacKinnon and co-workers used x-ray crystallography to solve the puzzle of how the inactivation gate works. They found that the protein tail at the end of the channel can enter the cytoplasmic part of the channel pore as an extended peptide to plug the channel— putting a cork in the bottle. Another gatekeeping puzzle being solved by the MacKinnon group is elucidation of the gating mechanism. Gating can be seen as the transduction of voltage or ligand binding to the switching on or off of Kϩ channels. This is an area of active investigation that is crucial to understanding channel function in cells (e.g., nerves and muscle) and how certain pharmaco- logic agents work. In a landmark series of papers, MacKinnon and co-workers provided the first structural views of how gating works. Their observations have not only shown that the gate is separate from the selectivity filter, but they also provided new ideas of how unexpected conformational changes can account for channel opening or closing. An important consequence of the separation of the selectivity filter and gates permits conformational changes in the protein to open or close the channel without altering its ability to discriminate among ions. All is, however, not so simple. In ClϪ channels where gating is closely tied to ClϪ ion conduction. MacKinnon and co- workers sought to determine the structural basis of gating in ClC ClϪ channels using the same type of x-ray crystallographic approach that had been successful with Kϩ channels. ClC channels permit passage of ClϪ ions across biologic membranes that govern the electrical activity of excitable cells like muscle and certain neurons. ClC channels also play critical roles in ClϪ absorption in the loop of Henle and distal tubule Roderick MacKinnon, MD (mutations in ClC-Kb or its accessory subunit, barttin, cause Bartter syndrome) and are important to endocytic processes in the proximal tubule (e.g., mutations in ClC5 cause Dent dis- fundamental question in biology. It turns out that a short and ease with a low molecular weight proteinuric and Fanconi-like narrow region of the outer pore, the selectivity filter, provides phenotype). MacKinnon and colleagues provided the first 3-D oxygen atoms on carbonyl side chains of amino acids that view of a ClC channel from the bacterium, Escherichia coli. interact with Kϩ ions in a way that is virtually identical to This has provided a fascinating look at how this channel selects Ϫ water oxygen molecules that form the hydration shell around and conducts Cl ions. When comparing the selectivity filters ϩ Ϫ the Kϩ ion in free solution. This narrow passageway is just the of K and Cl channels, MacKinnon recognized a fundamen- right size to coordinate best with Kϩ, but not Naϩ, ions— tal conserved principle from a chemical point of view. In both accounting for selectivity. In addition, the portion of the trans- channels, charges on alpha-helices (oxygen in Kϩ channels and membrane pore beyond the outer selectivity filter is a water- nitrogen in ClϪ channels) are used to stabilize the respective filled cavity so that the majority of the transit through the ion in the pore. greasy membrane is facilitated. This beautiful design allows These very exciting and fundamental discoveries by Roderick for both discrimination among ions and the rapid transit of ions MacKinnon and colleagues have not only provided a new molec- across the membrane. ular understanding of channel function, but they should also In electrically active cells like neurons, channels have a promote discovery of new channel-altering drugs and how they gatekeeper function that can shut down ion conduction, a work. Moreover, we will begin to understand how mutations that process called inactivation. Inactivation is critical for defining alter the amino acid sequence of these channel proteins modify the shape of action potential that mediates transmission of channel and cell function in inherited disorders.