Faculty

Curriculum Vitae

• B.A., University of Rochester, Phi Beta Kappa, 1973
• Ph.D., Molecular Biology, Washington University, 1982
• Postdoc, Department of Medicine (with Maria Buse), Medical University of S. Carolina, Charleston, SC, 1982-1983
• Postdoc, Department of Biological Sciences (with Elizabeth Jones), Carnegie Mellon University, 1983-1985
• Research Assistant Professor, Department of Biological Sciences, Carnegie Mellon University, 1985-1988
• Assistant Professor, Department of Molecular Biology and Microbiology, Case Western Reserve University School of Medicine, Cleveland, OH, 1988-1996
• Associate Professor, Department of Molecular Biology and Microbiology, Case Western Reserve Univ. School of Medicine, Cleveland, OH, 1996-2003
• Associate Professor, Department of Molecular and Cellular Pharmacology, University of Miami Miller School of Medicine, Miami, FL, 2003-2005
• Professor, Department of Molecular and Cellular Pharmacology, University of Miami Miller School of Medicine, Miami, FL, 2005-present
• Director, MD/PhD Program, University of Miami Miller School of Medicine, Miami, FL

Research Interests

Our laboratory is interested in the sorting and transport of proteins along the endocytic and secretory pathways. Movement along this elaborate membrane system is achieved by the budding, transport and fusion of membrane vesicles. We are trying to identify and characterize the coat proteins and other factors required for vesicle formation and selection of specific cargo molecules. One major class of coats includes clathrin and its associated adaptor proteins (AP's). These are involved in uptake of nutrients and regulated internalization of many signaling receptors. They are also required for sorting from the trans Golgi to the lysosome and for proper processing of many secreted peptides and hormones found in endocrine and nervous tissues. Defects in binding of clathrin and/or AP's to specific membrane components is the cause of a number of human diseases, including familial hypercholesterol anemia, resulting from defects in uptake of cholesterol into cells, and Hermansky-Pudlak Syndrome (HPS), a lysosomal disorder. Our lab studies clathrin-mediated transport in yeast. This model eukaryote has been extremely powerful for analysis of clathrin's role in membrane transport because it is highly amenable to molecular and genetic manipulation, and it permits detailed cell and biochemical analyses by taking advantage of sophisticated mutant analysis not available in most other eukaryotes.

Major Project 1. What are the functions of clathrin adaptors and accessory factors?

In yeast there are three complexes related to animal AP-1, AP-2 and AP-3. We previously found that AP-3 (the complex defective in some HPS patients) is required for a specialized sorting pathway to the yeast lysosome-like vacuole. We also found that AP's are not required for clathrin assembly and vesicle budding, bringing into doubt the long held dogma that AP's are absolutely required for clathrin function. This led to new studies to identify novel components of the clathrin-mediated transport pathway. One new clathrin interacting protein we identified is Sla2p, which is related to the huntingtin interacting protein, Hip1, and the related protein Hip1R. These are associated with clathrin at sites of endocytosis in animal cells. Sla2p is also crucial for endocytosis in yeast and associates with clathrin through the clathrin light chain (LC). We are generating mutants in the LC to characterize the Sla2p specific function of clathrin. In addition we are generating new clathrin mutants and using proteomics approaches to identify novel clathrin interacting factors.

Major Project 2: What is the role of the actin cytoskeleton and phosphorylation in endocytosis?

We are studying a novel protein, Scd5p, which is involved in regulating endocytosis and actin organization. Scd5p binds type one protein phosphatase (PP1), potentially serving as a targeting subunit for PP1. In addition, Scd5p is regulated by phosphorylation, including by a relatively new kinase family called ARKs, actin regulating kinases. Two mammalian kinases related to yeast ARKs, AAK1 and GAK, are directly involved in clathrin mediated transport in animal cells. We are trying to understand how phosphorylation by ARKs regulates Scd5p and how Scd5p/PP1 regulates actin organization during endocytosis. These studies are serving as a paradigm for the phosphoregulation of the endocytic machinery during synaptic vesicle recycling in the nerve synapse, which is regulated by CDK5 and the phosphatase calcineurin.

Major Project 3: Use of live cell imaging to examine the dynamics of endocytic vesicle formation.

Over the last 10-15 years many of the protein and lipid components needed to form an endocytic vesicle were identified, and many of their physical interactions have been characterized, but there was little information on how these components dynamically participated in the endocytic process. The use of variants of green fluorescent protein to tag factors involved in endocytosis and visualize their assembly and disassembly by live cell imaging has provided a remarkable new tool to understand the dynamic formation of individual clathrin-coated vesicle. By applying this technology we have shown for the first time in yeast that clathrin is at the cell cortex and recruitment of clathrin by AP180 and Epsin adaptors is one of the earliest steps in endocytic vesicle formation. Because of the easy manipulation of yeast we can now examine in detail the effects of specific mutations on the endocytic process and begin to dissect the specific roles of each factor. Currently we are focused on understanding the early stages in clathrin coated vesicle formation and the role of the Sla2p-LC interaction. Furthermore we are examining how phosphorylation by ARK kinases and dephosphorylation by Scd5/PP1 regulates the dynamics of endocytosis.

Major Project 4: Isolation of small molecule inhibitors of fungal growth from marine and aquatic microorganisms

We are collaborating with scientists at the University of Miami Rosenstiel School of Marine and Atmospheric Science and Florida International Univ. to perform chemical genomic screens to isolate small molecule inhibitors of fungal growth from marine and aquatic microorganisms. We are taking advantage of the unique molecular genetic tools in yeast to identify cellular targets of inhibitory toxins.

Research is funded by:

NIH/NIGMS R01 GM55796

Title: Role of Clathrin and Actin in Endocytosis in Yeast
Role: Principal investigator

Recent Publications

T. Newpher, F.Z. Idrissi, M.I. Geli, and S.K. Lemmon. “Novel Function of Clathrin Light Chain in Promoting Endocytic Vesicle Formation”, Mol. Bio. Cell. 17: 4343-4352 (2006)

T. Newpher and S.K. Lemmon. Clathrin is important for normal actin dynamics and progression of Sla2p-containing patches during endocytosis in yeast, Traffic 7: 574-588 (2006).

J. Chang, K. Henry, M. Geli, and S.K. Lemmon. “Cortical Recruitment and Nuclear-Cytoplasmic Shuttling of Scd5p, a Protein Phosphatase 1 Targeting Protein Involved in Actin Organization and Endocytosis”, Mol. Biol. Cell 17: 251-262 (2006).

T. Newpher and S.K. Lemmon. “In Vivo Dynamics of Clathrin and Adaptor-Dependent Recruitment to the Actin-Based Endocytic Machinery in Yeast”, Dev. Cell. 9: 87-98 (2005).

L. Cheng, S.K. Lemmon and V. Lemmon. "RanBPM is an L1-interacting Protein That Regulates L1-mediated MAPK Activation", J. Neurochemistry 94: 1102-1110 (2005).

K. Henry, K. D’Hondt, J. Chang, D. Nix, M.J.T.V. Cope, C.S.M. Chan, D.G. Drubin, and S.K. Lemmon, “The Actin Regulating Kinase Prk1p Negatively Regulates Scd5p, a Suppressor of Clathrin Deficiency, in Actin Organization and Endocytosis”, Curr. Biol. 13:1557-1562 (2003).

J. Chang, K. Henry, B. L. Wolf, M. Geli, and S.K. Lemmon, “Type 1 Protein Phosphatase Binding to Scd5p is Important for Regulation of Actin Organization and Endocytosis in Yeast”, J. Biol. Chem. 277: 48002-48008 (2002).

K. Henry, K. D’Hondt, J. Chang, T. Newpher, R.T. Hudson, K. Huang, H. Riezman and S.K. Lemmon, “Scd5p and Clathrin Function are Important for Cortical Actin Organization, Endocytosis and Localization of Sla2p in Yeast”, Mol. Biol. Cell, 13: 2607-2625 (2002).

D. Gelperin, L. Horton, J. Hensold, A. de Chant and S.K. Lemmon, “Loss of Ypk1 Function Causes Rapamycin Sensitivity, Inhibition of Translation Initiation and Synthetic Lethality in 14-3-3-deficient Yeast”, Genetics 161: 1453-1464 (2002).

A.W. Schaefer, Y. Kamei, H. Kamaguchi, E.V. Wong, I. Rapoport, T. Kirchhausen, C.M. Beach, G. Landreth, S.K. Lemmon, and V. Lemmon, “L1 Endocytosis is Controlled by a Phosphorylation-Dephosphorylation Cycle Stimulated by Outside-in Signaling by L1”, J. Cell Biol. 157: 1223-1232 (2002).

D. Gelperin, L. Horton, J. Beckman, J. Hensold and S.K. Lemmon, "Bms1p, a novel GTP-binding protein, and the related Tsr1p are required for distinct steps of 40S ribosome biogenesis in yeast", RNA 7: 1268-1283 (2001).

X. Pan, P. Roberts, Y. Chen, E. Kvam, N. Shulgal, K. Huang, and S. K. Lemmon, and D.S. Goldfarb, "Nucleus-Vacuole Junctions in Saccharomyces cerevisiae are Formed Through the Direct Interaction of Vac8p and Nvj1p", Mol. Biol. Cell 11: 2445-2457 (2000).

K. Huang, K. D'Hondt, H. Riezman, and S.K. Lemmon, "Clathrin Functions in the Absence of Heterotetrameric Adaptors and AP180-related Proteins in Yeast", EMBO J. 18: 3897-3908 (1999).

J.J. Fabrizio, G. Hime, S.K. Lemmon and C. Bazinet, "Genetic Dissection of Sperm Individualization in Drosophila melanogaster", Development 125: 1833-1843 (1998).

J.D. Stepp, K. Huang and S. K. Lemmon, "The Yeast AP-3 Complex is Essential for the Efficient Delivery of Alkaline Phosphatase by the Alternate Pathway to the Vacuole", J. Cell Biol. 139:1761-1774 (1997).

H.R. Panek, J.D. Stepp, H.M. Engle, K.M. Marks, P. Tan, S.K. Lemmon and L.C. Robinson," Suppressors of YCK-encoded Yeast Casein Kinase 1 Deficiency Define Four Subunits of a Novel Clathrin AP-like Complex”, EMBO J. 16: 4194-4204 (1997).

K. Huang, L. Gullberg, K. Nelson, C. Stefan, K. Blumer and S.K. Lemmon, “Novel Functions of Clathrin Light Chains: Clathrin Trimerization is Defective in Light Chain-Deficient Yeast”, J. Cell Science 110: 899-910 (1997).