Profile of Sabeeha Merchant
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PROFILE PROFILE Profile of Sabeeha Merchant Tinsley H. Davis Science Writer Organic molecules dominate the biologi- major from chemistry to molecular biology. cal world, yet many of life’s critical mo- “It was a new major at the time that most lecular reactions depend on metals for small schools didn’thave.” catalysis. Sabeeha Merchant, a professor Merchant entered the University of Wis- of biochemistry at the University of Cal- consin, Madison, in 1978 and discovered that ifornia, Los Angeles (UCLA), studies how taking courses in a wide range of subjects cells keep track of crucial trace nutrients provided an enjoyable change from her early like iron and copper. Her early work on schooling, with nonscience courses like logic Chlamydomonas reinhardtii, a single-celled shaping her analytical skills. “I had to take all alga that, like all plants, primarily uses a of the biology that I avoided in high school.” copper-based electron transporter during This time, however, she found courses that photosynthesis, revealed that the organism piqued her interest in the chemistry of liv- engages an alternate, iron-dependent mol- ing cells. “That’swhenthingsclicked,” says ecule when copper availability is low (1). Merchant, a chemist at heart. Merchant, who Merchant, elected to the National Acad- still did not think of science as a career, also emy of Sciences in 2012, says, “I always appreciated being exposed to research. Just get asked, ‘Why have two pathways?’” In before graduating, in 1979, she took a job as her Inaugural Article, Merchant provides biochemist Henry Lardy’s secretary in the experimental support for plastocyanin as a Enzyme Institute. During her tenure, Mer- Sabeeha Merchant. Image courtesy of Reed de facto copper storage molecule. The pro- chant began to perform experiments in the Hutchinson. tein is active during times of copper suffi- laboratory. ciency but gets degraded to liberate its Eventually, with Lardy’s insistence, Mer- ’ copper stores for other essential needs chant applied and gained acceptance to bio- spent the time when she wasn t setting up the when the backup pathway is initiated in chemistry graduate programs across the new laboratory in the library reading papers. “ copper-deficient environments (2). country. She chose to stay at Wisconsin be- I always tell students that going to the li- cause it seemed student centered. Merchant brary is a really important part of being ’ There s No Escaping Biology undertook her doctoral research with Bruce a scientist,” she says. “Though now it just Merchant,bornin1959,grewupinMumbai, Selman, who had just started studying comes to you on your computer.” During India, where students often choose an area of Chlamydomonas reinhardtii, a ubiquitous these hours in the library, Merchant found specialization by age 12. Her focus on science single-celled alga, relatives of which grow a 1978 paper by biochemist Paul Wood proved the result of aptitude rather than in- even in snow or sewage. The cells’ ability to showing that Chlamydomonas preferentially “ terest. Which 12-year-old knows what he or grow and build a photosynthetic apparatus uses plastocyanin, a copper protein, in the ” “ she wants to do? she says. Only a few, and in the absence of light makes it an attractive photosynthetic electron transport chain when the rest of us took so-called aptitude tests system to study the components of photo- copper is available and switches to cyto- c in various subjects. I tested positive for synthesis because the pathway is not re- chrome 6, an iron-containing protein, if both humanities and science and ended up quired for survival. Merchant purified and copper is low (5). ’ in the science track because it didn thave characterized Chloroplast Coupling Factor Intrigued, Merchant approached Bogorad ” enough girls. 1, a critical component of the photosynthetic with her interest in studying how cells detect An excellent student, Merchant liked pathway that synthesizes ATP, the cellular copper levels. “Nobody was studying metals chemistry, math, and physics but not biology. energy currency, from the proton gradient as regulators at that time,” says Merchant. “ I could handle the earthworm dissections (3) and showed that the α β subunit stoi- She eagerly undertook the project and made ” 3 3 but not the cockroach or frog. She started chiometry of bacteria and mitochondria is inroads in just a few months, generating university at age 15 in India, although her conserved in chloroplasts as well (4). monospecific antibodies against both pro- family soon relocated to the United States teins in rabbits; 30 years later, Merchant when her younger brother graduated from Fresh Opportunity distributes the antibodies globally and uses high school and decided to pursue engi- For her postdoctoral fellowship, Merchant them in her own work. neering. The family moved near relatives to wanted to learn molecular biology and chose Merchant showed that RNA for C. rein- Whitewater, WI, going from “acityof10 to work with Lawrence Bogorad at Harvard hardtii plastocyaninisalwayspresentbutthe million to 10,000, most of those students. We University. In 1984, she began working on had nothing to do,” she recalls. “Iquickly light-regulated gene expression, but 4 months ” This is a Profile of a recently elected member of the National figured out that I needed to transfer. To later, a devastating fire destroyed the labo- Academy of Sciences to accompany the member’s Inaugural Article convince her mother, Merchant changed her ratory. With no projects running, Merchant 10.1073/pnas.1422492112. www.pnas.org/cgi/doi/10.1073/pnas.1500798112 PNAS Early Edition | 1of2 c proteinisonlyproducedwhencopperis and cytochrome 6 accumulation, albeit via intensive fractionations required to separate available. Pulse chase experiments revealed different mechanisms (9). Through the 1990s, them. Merchant designed the experiment to that plastocyanin is also translated all of the Merchant, with departmental colleague Todd replicate what algal cells might experience in time but degraded in low copper conditions Yeates, focused on structural and genetic nature during an algal bloom, from the initial c (6). She found that cytochrome 6 RNA, aspects of plastocyanin and its iron-contain- phase, with low cell density and high copper c however, is only present in copper-deficient ing alternate cytochrome 6 in parallel. In availability, to high cell density and copper cells (1). This alternate, iron-based protein addition to solving the crystal structures of scarcity. The team used cell fractionation and only appeared when the cell called for rein- both proteins (10, 11), they teased apart the inductively coupled plasma mass spectrom- forcements. Chlamydomonas had two means genetic pathways to study the proteins’ bio- etry to obtain the elemental profile of cells to the same end, a concept that would pro- synthesis and expression. and cell fractions in each nutrient profile and vide fruitful investigative avenues for years to Merchant now estimates that copper rep- showed that copper from the plastocyanin “ ’ come. The concept of a backup protein did resents only 25% of the laboratory s efforts, fraction moves to the cytochrome oxidase ” not exist at that time, she says. having broadened her focus to explore iron fraction when cells grow in copper-deficient sensing and the role of iron limitation on environments (2). The work underscores her Separating Contamination “ photosynthesis (12). Iron is much more thinking that plastocyanin functions as a Merchant joined the UCLA faculty in 1987. ” “ fundable than copper work, she says. There copper reservoir and perhaps gives cells a Merchant recalls her relief that the offer at are100timesmoreironproteinsthancop- UCLA included a water purification system, competitive advantage in a copper-deficient per, therefore there are more diseases and environment. uncommon at the time, to filter out trace nutritional issues associated with iron-defi- “ metals. Studying metal nutrient deficiency My research has definitely taken a bi- ciency.” The difference is attributable to the ” requires following strict protocols to ensure ’ ological turn. Now we do a lot of genomics, elements representation as life evolved on she says. She led a team that sequenced the that no stray metal ions contaminate the earth. “Copper is new to biology,” explains “ Chlamydomonas genome in 2007, and her experiments. It takes me a long time to train Merchant. “It’s younger than iron,” having ” laboratory is working onsequencingextrem- students and post docs, she says. Iron is become a bioavailable redox agent only after “ ’ ophiles (14). Extremophiles’ responses to easy to get rid of because it s not bioavail- theintroductionofoxygenintotheearth’s ” metal availability pique Merchant’s interest able, but other metals are difficult. Most changing atmosphere, an event that also because many of these organisms live at water pipes are copper, hence the need for transformed iron into its less bioavailable extremes of the pH scale, and metal avail- ultrapure water. oxidized form (13). To mitigate contamination, Merchant’s ability is exquisitely pH dependent. She laboratory also acid washes all glassware with Shifts in Evolution expects the work will keep her occupied, high concentrations of hydrochloric acid, The relative paucity of copper proteins, how- especially as she explores extremophiles dislodging metal ions from the surface with ever, makes copper attractive for studying living in nutrient-scarce environments like protons. They