Structural Biology of Plant Lectins and Macromolecular Crystallography in India
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REVIEW ARTICLE Structural biology of plant lectins and macromolecular crystallography in India Nukathoti Sivaji, Kaza Suguna, Avadhesha Surolia and Mamannamana Vijayan* Molecular Biophysics Unit, Indian Institute of Science, Bengaluru 560 012, India they were described as phytohemagglutinines. Many Earnest attempts at initiating macromolecular crystal- lography, which is central to modern biology, started plant lectins have specificity for different blood groups. Subsequently, lectins were found in animals, algae, bacte- in India in the 1970s. The most important component 4 of the efforts was on plant lectins. The lectin pro- ria, fungi, yeast and viruses as well . Most recently, in gramme was continuously and almost exclusively sup- our laboratory, we have established the presence of lec- ported by the Department of Science and Technology, tins in archea. Thus lectins exist in all the three domains Government of India. Lectins are quintessential of life, viz. eukaryotes, eubacteria and archea. It would carbohydrate-binding proteins and have acquired therefore appear that lectins are a group of important pro- considerable importance in relation to biological rec- teins which presumably originated before the three ognition at cell surface. The plant lectin programme at domains diverged. Bengaluru spanned 40 years and was instrumental in Sugar and their polymers, viz. oligosaccharides and making significant contributions to glycobiology. The polysaccharides, abound in nature. They often occur in work has also produced important results pertaining to protein folding, quaternary association and strate- combination with proteins and lipids. Cellulose, a poly- gies for generating ligand specificities. The role of the mer of the sugar glucose, is the most abundant biomole- lectin programme in mentoring scientists has been cule in nature as it forms a structural component of important and many of the senior macromolecular plants. There are also many other polysaccharides with crystallographers in the country were initially trained important structural roles. Sugars can polymerize in dif- under this programme. Studies on plant lectins led to ferent ways. For instance, some sugars polymerize in those on mycobacterial and archeal lectins. The lectin ways different from that observed in structural polysac- work has also served as a springboard for the initia- charides, to yield important food materials, e.g. sucrose, tion of several other long-range programmes which, in starch, etc. in the form of carbohydrates. In spite of their addition to yielding important results, have also importance as structural materials and components of helped in training scientists, many of whom are again food, they received much less attention than proteins and leaders of structural biology in India. Perhaps, major- ity of macromolecular crystallographers in India are nucleic acids. The situation changed with the realization those who have been trained at Bengaluru and their that most of the recognitive processes on the cell surface descendants. Particularly in recent years, there has are mediated by sugars. Consequently, lectins which are been an influx of scientists trained in other centres, quintessential sugar-binding molecules with recognitive who have added vibrance to the macromolecular crys- functions, began to receive considerable attention. To tallography community. The community is now start with, much of this attention was on plant lectins. It reasonably coherent, with constructive interactions is in these early stages that we initiated our work on plant among its members, and with several common lectins which enabled us to make important, globally programmes and shared facilities. competitive contributions in the area. Furthermore, struc- tural biological studies on plant lectins turned out to be Keywords: Ligand specificity, macromolecular crystal- an important vehicle for the initiation and development of lography, plant lectins, protein folding, quaternary asso- macromolecular crystallography in India. In what ciation. follows, we outline the historical context and review our major findings in that context. We also describe briefly, LECTINS are often described as multivalent carbohydrate- succeeding and parallel efforts at Bengaluru on related binding proteins of non-immune origin, which specifically and other systems. recognize different sugar structures1–3. They were origi- nally discovered in plants and their best-known property Background was the ability to agglutinate red blood cells. Therefore, India has had a distinguished tradition in X-ray crystallo- *For correspondence. (e-mail: [email protected]) graphy. G. N. Ramachandran (G.N.R.) and his colleagues 1490 CURRENT SCIENCE, VOL. 116, NO. 9, 10 MAY 2019 REVIEW ARTICLE gave a head start to the country in structural biology, in tin could not be crystallized. We had better luck with addition to contributing to the foundations of crystallo- peanut agglutinin (PNA), which readily crystallized. A graphy. However, most of their efforts in structural crystallization note outlining this work was published in biology were computational in nature. G.N.R. very much early 1982 (ref. 9). In the meantime, M.V. had initiated a wanted to initiate macromolecular crystallography, the programme on the hydration of proteins and its conse- most important component of structural biology in India, quences based on water-mediated transformation. Yet but the time was not ripe for doing so. A few Indians another major programme at MBU involved fibre diffrac- were involved in early macromolecular crystallography tion studies on DNA by V. Sasisekharan in collaboration studies abroad. However, one of us (M.V.) was the first with S. K. Brahmachari and others. The Department of trained macromolecular crystallographer to return home. Science and Technology (DST), Government of India When M.V. returned to India and joined the Indian In- (GoI) had by then formulated their Thrust Area Pro- stitute of Science (IISc), Bengaluru in 1971, the available gramme. In 1982, V. Sasisekharan, M.V., A.S. and Bra- resources were totally inadequate to start structural work hamachari submitted, a comprehensive X-ray project on macromolecules like proteins. The interactions involving work on DNA and proteins. After much delibe- between crystallographers and biochemists were also ration, the project was sanctioned in 1983. In the course inadequate. Therefore, much of his early work was on of discussions, it was agreed, particularly at the instance crystalline complexes involving amino acids and pep- of then DST Secretary, S. Varadarajan and Chairman of tides, with emphasis on aggregation and interaction the Programme Advisory Committee, N. Seshagiri, that patterns. This effort turned into a major programme with the Bengaluru group should function as a national its implications for supramolecular association and chemi- nucleus for the development of macromolecular X-ray cal evolution. However, he was constantly on the lookout crystallographic studies in India. The handsome grant for avenues to initiate macromolecular crystallographic provided by DST in 1983 was perceived as a first step in studies. It is in this context, that interaction with A.S. be- this direction. Within a few years, the work on fibre dif- came important. In 1978, M.V. was a comparatively fraction was wound up. Funds for studies based on water- young faculty member in the Molecular Biophysics Unit mediated transformations were obtained from other (MBU) at IISc. By then, A.S. had obtained his doctorate sources. The lectin programme remained with DST. working on the biochemistry and physical chemistry of Overall support for the X-ray facility at MBU and that for plant lectins and was at the Indian Institute of Chemical the lectin programme has been provided by DST from Biology, Kolkata. Their meeting at Hyderabad was 1983 to date. Indeed, during a period, the programme on arranged during late 1978 by a common friend, S. K. lectins used to be projected as a flagship programme of Poddar, who was a faculty member of the Department of DST. Biochemistry at IISc. That was the beginning of structur- In the early 1980s, the PNA molecule was large even al biology studies on plant lectins in India. Forty years by global standards. The available experimental and later, in 2018, the very last structural biology paper of computational facilities in India were inadequate to deal (M.V. and A.S.) on plant lectins appeared in Glycobiology5. with the problem. Furthermore, the unusual nature of the During these four decades M.V. and A.S. have collabo- quaternary association of the protein, elucidated subse- rated on plant lectins. In the meantime, A.S. joined MBU quently, also contributed to the difficulty in solving the as a faculty member in 1981. That naturally further facili- structure. It was established that PNA is a tetramer like tated the collaboration. K.S. joined MBU in 1988 with ConA. Most of the early efforts were to solve the struc- considerable experience in work on plant lectins in a US ture using molecular replacement on the basis of this laboratory. Since then, the three of us (M.V., A.S. and similarity and the known substantial sequence identity K.S.) formed a core group in the structural biology between PNA and ConA10. These efforts failed. The suc- programme at Bengaluru on plant lectins. Of course, as cess during the first 10 years of lectin crystallography at indicated later, many other distinguished scientists contri- Bengaluru was in crystallizing and characterizing