EXROP Projects: Stephen C. Harrison

Stephen C. Harrison

Summary

Stephen Harrison's laboratory studies the atomic structures of macromolecular assemblies, such as viruses and protein/nucleic-acid complexes, to understand how they function in cells.

Summer Lab Size: 20
Local Summer Program: Harvard EXROP Summer Experience
Program Dates: June 10-August 16, 2013 (Dates for 2014 will be similar)

Virus Structure and Cell Entry

From the perspective of structure and entry mechanisms, viruses that infect humans or other vertebrates fall into two broad classes: those with lipid-bilayer membranes of their own (e.g., influenza virus, HIV), called "enveloped" viruses, and those without lipid membranes, called "nonenveloped" viruses. Enveloped viruses enter cells by a process in which the viral membrane fuses with a cellular membrane, depositing the internal contents of the virus, including its genome, into the cytoplasm of the new host cell.

Projects for the summer will involve studies of the molecular mechanism of membrane fusion as facilitated by viral "fusion proteins" (such as the hemagglutinin of influenza virus or the envelope protein of HIV). The viruses we are studying include influenza virus, dengue virus, and HIV. A student who participates will be carrying out biochemical, molecular-biological, and physicochemical experiments, probably involving one of these viruses in noninfectious form (e.g., virus-like particles). The student will learn about mechanistic approaches to problems in cell biology and about basic virology and virus-cell interactions.

Antibody Affinity Maturation

Antibodies are the main line of defense against many viral pathogens. Initial exposure to a virus or to a vaccine induces proliferation of B cells that recognize viral antigens, but the first response generates relatively weakly binding antibodies. Continued proliferation in the presence of antigen, together with a process of somatic hypermutation that randomly alters the variable regions of the encoded antibody molecule, leads to selection of B cells that secrete more tightly binding antibodies.

Recently developed technologies allow one to obtain the sequences of the variable regions of antibody heavy and light chains from many individual B cells in a donor sample. In suitable cases, such as influenza vaccination, it is possible to reconstruct the "evolutionary tree" of a set of such antibodies, deducing the amino acid sequences of the progenitor antibody (i.e., the antibody produced by the initially stimulated B cell) and of various intermediates between the progenitor and the mature antibodies present at the time a blood sample is obtained. Structural, biochemical, and computational analysis of these lineages give us a picture of protein evolution over a short period of time, in response to a defined selective pressure. This information may be useful for new modes of vaccine design.

A student who participates will be carrying out biochemical and molecular-biological experiments to express and purify suitable antibodies for structural and physicochemical studies.

Scientist Profile

Investigator
Harvard Medical School
Biochemistry, Structural Biology