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A Collaboration Built Upon Structures
Some time in late 1999, Finn wandered down to the laboratory
of TSRI Professor John Johnson, whom he had recently met.
As Finn describes it, the two of them then spent a "golden
afternoon" in Johnson's laboratory looking at the viral structures
that Johnson had been working on. Among these structures was
the one of cowpea mosaic virus, an icosahedral RNA virus that
infects the plant that produces black-eyed peas.
"I learned from Jack that you could make these [viruses]
in gram quantities," says Finn. The structures of the virus
particles were known from Johnson's work, and Finn realized
as he was fiddling with some of the three-dimensional models
that afternoon that they could be just the building blocks
he was looking for. Finn saw these virions as "supramolecular"
chemical reagents that can be manipulated to display a number
of interesting molecules by attaching other chemicals to the
side chains of the viral component proteins.
"At that moment, I proposed to Jack that we collaborate,"
says Finn.
And collaborate they did. As a member of The Skaggs Institute
for Chemical Biology, Finn used funding that was provided
to him by The Skaggs Institute for Research to get the project
started. Johnson and TSRI Assistant Professor Tianwei Lin
helped Finn's group get a handle on the molecular biology
of the viruses, showing him how to manipulate their protein
sequences and to express and purify them, and Finn contributed
his organic synthetic experience to Johnson's virions, designing
ways to attach new molecules to their surfaces.
The molecular biology of the virions can be tweaked so as
to provide different kinds of "hooks" onto which different
chemicals can be attached. The attachments are made after
the particles are harvested, intact, from infected plants,
and these particles are so sturdy that the chemistry can be
done over a wide range of pH levels, temperatures, and organic
solvent concentrations.
These attachments are made via lysine or cysteine side chains
on the subunits of the proteins that come together to make
the viral shell. Since there are multiple protein subunits
and potentially multiple exposed lysine or cysteine side chains,
multiple copies of the added chemical can be attached.
In so doing, it is possible to produce materials with a
number of different properties and a variety of potential
uses. The fertile ground created by this intersection of chemistry
and biology was recognized in 2001 by the David and Lucile
Packard Foundation, which made Finn and Johnson the recipients
of its Interdisciplinary Science Program award in a national
competition.
Finn's collaboration with Sharpless also allowed him to
pioneer click chemistry as a method for making attachments
to biological molecules, using viruses as the test case. This
has been picked up by a number of laboratories, including
that of Cell Biology and Chemistry Associate Professor Ben
Cravatt, for making bonds in and around cells.
Drug Delivery and Materials Design
One of the most obvious uses of virus particles is biomedical—the
delivery of a drug to a particular tissue or cell type in
the body, for instance. "Can you take this big particle, steer
it to a particular cell type, and deliver a payload?" Finn
asks.
The first step in this process, he says, is chemical sensing,
or targeting a particle to a certain part of the body. For
this targeting, Finn collaborates with Cell Biology Assistant
Professor Marianne Manchester, who has an adjacent office
and shares laboratory space in the CIMBio building. Manchester
has specialized in following dye-decorated virus particles
through whole animals and tailoring them genetically to find
particular tissues. The goal of their collaboration is to
turn the viruses into molecules that could report on disease
states and perform drug delivery.
"I think we are getting pretty close," Finn says, adding
that the next step is extending the technology to deliver
a payload.
For the delivery, Finn works with Department of Cell Biology
Chair Sandra Schmid to characterize the receptor-mediated
endocytosis of virus particles with carbohydrates displayed
on their surfaces. The hope is to be able to induce the cells
to take up the virus particle selectively. One key to this
is the fact that the virions are polyvalent, and multiple
copies of some endocytosis "effector" molecule can be displayed
on their surface in well-defined patterns and distances.
Finn is also working with Associate Professor Glen Nemerow
in the Department of Immunology to try to design plant virus
particles that mimic adenovirus, the virus that causes the
common cold, which Nemerow studies. Adenovirus is already
adept at getting into cells through a complicated binding
and entry mechanism.
In a different twist, Finn and his laboratory are trying
to use Johnson's virions to make new materials, and they have
made progress in getting the virions to assemble themselves
into supermolecular assemblies, aggregates, and other nanostructures
by putting different molecules on the outside of the virions.
"This is the first step for us," says Finn, who adds that
his goal is to be able to program the virions to make the
emergent assemblies he desires.
To test this, graduate student Erica Strable, a Ph.D. candidate
in TSRI's Kellogg School of Science and Technology who is
a joint member of the Finn and Johnson laboratories and is
funded by the La Jolla Interfaces in Science Program directed
by TSRI Professor Libby Getzoff and UCSD Professor Jose Onuchic,
chemically attached DNA oligonucleotides to the outside of
the virus and created different pools of such oligo–virus
particles with complimentary bases. Depending on where the
oligos were placed on the virus, Strable found that she could
assemble the pieces into two-dimensional arrays or three-dimensional
aggregates in a temperature-sensitive fashion (since high
temperature melts DNA).
He also has been experimenting with assembling these particles
by attaching antibodies to some and antigen to others, by
using metal interactions and attaching sugars to some virions
and carbohydrate binding proteins to others. All of these
have potential uses in biology and nanotechnology, applications
that Finn is currently exploring with his many collaborators
"The best part about being here," he says. "is all the fabulous
people who are in my group and around the institute. I delight
in being a mentor to the wonderful students and postdocs that
I find here at Scripps, both in my group and in the graduate
program."
"And," says Finn, "I'm having the time of my life."
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