Wednesday, April 25, 2018 at 12:20pm
Professor, PPPMB, Cornell University
Sondra Lazarowitz, Ph.D., is a Professor in the Department of Plant Pathology at Cornell University, a position she has held since 1998. LazarowitzÕs lab conducts research on how the interactions between viruses and their host plants lead to disease, using several model viruses and the model plant Arabidopsis thaliana. She uses the approaches of molecular genetics, cell biology and genomics to understand how plant viruses use the cellular trafficking machinery to spread from cell-to-cell and invade the host plant. Research in her lab defined, in molecular terms, how geminiviruses move within and between plant cells, and has led to the identification of the first nuclear export signal in a plant protein and to the discovery in plants of a class of proteins, which were thought to only be found in the animal nervous system (synaptotagmins). Lazarowitz has also been active in K-12 science outreach since 1992, having been Program Director of Howard Hughes Medical Institute programs in Undergraduate Biology and Precollege Outreach at Cornell (1999-2004) and the University of Illinois (1992-1998). Lazarowitz, together with George Keiffer and Claudia Washburn, created the Prairie Flowers Program, which has fostered systemic change in middle school science education in rural Illinois. Together with Professor Jerry Uhl at Illinois, she created BioCalc, an innovative introductory calculus course for biology majors at the University of Illinois. Her education and outreach programs have been highlighted in "Beyond Bio 101: The Transformation of Undergraduate Biology Education", the Howard Hughes Medical Institute Bulletin, and on the Discovery Channel. Dr. Lazarowitz holds a Ph.D. in virology and cell biology from The Rockefeller University, and earned a S.B. in life sciences from the Massachusetts Institute of Technology.
Our overall goal is to identify the molecular and cellular events, which underlie the ability of viruses to infect and spread within a plant to cause disease. Because viruses are obligate intracellular parasites, defining these events can not only lead to the rational development of anti-viral strategies, it also provides insights into how the host regulates gene expression, signal transduction and macromolecular trafficking. For plant viruses, an essential element of their disease potential is their ability to cross the barrier of the plant cell wall to invade the host. Our research has focused on a novel class of proteins encoded by plant viruses: ‘movement proteins’, which play a key role in coordinating replication of the virus genome with its directed transport to and across the plant cell wall. We use the model plant Arabidopsis thaliana and two model plant viruses: the DNA geminivirus Cabbage leaf curl virus (CaLCuV), and the RNA tobamovirus Tobacco mosaic virus (TMV). Employing a variety of approaches –– from molecular and classical genetics to biochemistry, structural biology, confocal imaging, robust transient expression assays and functional genomics –– we have established the current model for how geminivirus movement proteins act to transport the viral genome within and between cells.
Our research findings have lead us identify two essential macromolecular transport pathways, which are pirated by plant viruses to transport viral genomes, and to now focus on investigating the role of these trafficking pathways in plant growth and development, as well as in virus movement: (1) The role of vesicle trafficking in plant cell-cell communication; and (2) The regulation of nuclear import and export in plant cells.
Why study movement proteins?
Plant viruses must cross the barrier of the plant cell wall to move cell-to-cell and invade the host. To do this, they encode unique movement proteins, which are major determinants of virus host range and disease potential. Whereas movement proteins encoded by different viruses can be distinct in their sequences and vary in their specific functions, they ultimately target a common pathway to carry the viral genome across the wall: they alter plasmodesmata (Pd), complex transwall pores that connect adjacent plant cells. Indeed, movement proteins coordinate replication of the viral genome with its directed transport to and across the cell wall to invade the host. They do this by exploiting the cellular machinery for trafficking macromolecules. The final act of crossing the wall is preceded by regulated stages in which movement proteins, with their viral genome cargo, interact with the endomembrane system, the cytoskeleton and, in the case of geminiviruses, the nuclear import and export machinery. This makes them robust models to investigate macromolecular trafficking in plant cells and the regulation of plant cell-cell communication.
From geminivirus movement to vesicle trafficking and nuclear shuttling.
Our research has established the current model for how the two geminivirus movement proteins, NSP and MP, cooperate to transport the viral single strand DNA genome from its site of replication in the nucleus to, and across, the cell wall (see figure below). NSP (Nuclear Shuttle Protein) acts to import the virus genome into the nucleus for replication, and to export newly replicated virus genomes back to the cytoplasm. MP (cell-to-cell Movement Protein) traps NSP-genome complexes in the cytoplasm and directs these to the cell wall for movement, through what appear to be altered Pd, and into adjacent uninfected cells. NSP then targets the viral genome back to the nucleus to initiate new rounds of infection. Building on this knowledge, we have identified Arabidopsis proteins, which interact with NSP and MP. This has lead us to discover a family of proteins in plants, which regulate vesicle fusion and were thought to only be present in the animal nervous system: synaptotagmins. Our current research is focused on the mechanisms of nuclear shuttling and of vesicle trafficking in plant cells, in terms of how these pathways regulate plant growth and development, and how viruses pirate these pathways to infect the plant and cause disease.
The role of vesicle trafficking in plant cell-cell communication.
Synaptotagmins (SYTs), are a multigene family thought to be exclusive to animals due to their role in neurotransmitter release. They are calcium (Ca2+) sensors, which are proposed to regulate rapid and synchronous synaptic vesicle exocytosis and endocytosis. What functions do SYTs carry out in plant cells? How are these functions linked to plant virus movement and cell-cell communication via Pd? Answering these questions is one major focus of our lab. There are 5 SYTs (SYT A, B, C, D, E) encoded in Arabidopsis. Genome: SYTA regulates the cell-to-cell trafficking of the distinct movement proteins encoded by CaLCuV (MPCaLCuV) and TMV (MPTMV). Our current studies of SYTA have identified an endosome recycling pathway, which acts to ferry movement proteins and their virus genome cargos to Pd for transport across the cell wall. Our findings also suggest that SYTs play key roles in regulating both the cell-to-cell spread of most, if not all, plant viruses and cell-cell communication in plant development. Questions we are currently addressing include: (1) Does SYTA play a central role in the movement of most, if not all plant viruses? (2) Is it the only SYT with this role? (3) What are the functions of SYTs B, C, D and E in vesicular traffic? (4) Do other SYTs cooperate with SYTA in endosome recycling and/or virus movement? (5) What are the functions of SYTs in plant growth and development?
The regulation of nuclear import and export in plant cells.
We have visualized the nuclear shuttling of NSP by confocal microscopy, using a transient expression assay in plant protoplasts. These studies established that the mechanism of nuclear shuttling is highly conserved in plants, as well as in animal cells and yeast. NSP is a typical rapidly shuttling nuclear protein, which contains two classic basic nuclear localization signals. It also contains an essential leucine-rich nuclear export sequence, which suggests that it is exported by an exportin 1-type pathway. There are 7 exportin genes, 11 importin α genes, and 11 importin β genes in Arabidopsis. We are currently determining the structure of NSP itself, and bound to DNA, and examining the defective phenotypes in Arabidopsis lines, which are mutated in the importin α and exportin genes. These will provide the basis for our future studies, in which we: (1) identify the specific importin a proteins and exportin(s) that interact with NSP, and examine the structures of these complexes; and (2) in collaboration with investigators at Stony Brook, Ohio State Univ. and the Univ. of Kentucky, use functional genomics to map out the integrated network of pathways that comprise importins, exportins and nuclear pore proteins in Arabidopsis.