News and Publications
The Skaggs Institute For Chemical Biology
Scientific Report 1997-1998
Expression of Antibodies and Novel Enzymatic Functions in Algae
S. Mayfield, J. Kim, E. Efuet, R. Bruick, A. Lentz, P. Choi, J. Allen, C.
We express antibodies in algae for several reasons. For example, because
of the advent of bacteria resistant to antibiotics, alternative treatments are
essential to maintain the current standards of health care. Treatment of bacterial
infections with antibodies is quite effective, but the cost of producing antibodies
makes such treatment prohibitively expensive. Production of antibodies in algae
should reduce these costs significantly and make antibody therapy a practical
alternative to treatment with antibiotics. In addition, antibodies are protein
molecules that can bind other molecules, both complex and simple, with high specificity
and affinity. This attribute has made antibodies ideal molecules for a number
of biotechnologic uses, and this specific binding of antibodies has been exploited
by research groups to engineer antibody catalysts, including catalysts that do
not occur naturally.
Because antibodies are increasingly used as therapeutic and research tools,
the need to produce these proteins in pure form and in large quantities is obvious.
Plants or algae can be used to produce these pharmacologically important proteins
and enzymes on a large scale and in relatively pure form. Expression of antibody
catalysts in algae will enable us to introduce novel enzymatic functions into
these organisms to alter metabolites to produce new compounds.
Microalgae have several unique characteristics that make them ideal organisms
for the production of antibodies. First, unlike most organisms and cells currently
used to produce transgenic proteins, algae can be grown on a large scale in minimal
media (inorganic salts) with sunlight as the energy source. Second, plants and
algae have 2 distinct compartments, the cytoplasm and the chloroplast, in which
proteins can be expressed. The cytoplasm of algae is similar to that of other
eukaryotic organisms used for protein expression, such as yeast and insect cells.
Chloroplasts are unique to plants and algae, and proteins expressed in this environment
most likely will have properties different from those of cytoplasmically expressed
We engineered strains of the green alga Chlamydomonas reinhardtii to
express antibody genes in the chloroplast. We expressed a truncated version of
an antibody, known as single-chain antibody, that recognizes the tetanus toxin
and a second antibody that is a catalytic aldolase. We also constructed a vector
that contains a larger antibody composed of both a light-chain protein and a
heavy-chain protein. Both the single-chain antibody and the larger antibody are
capable of binding tetanus toxin, and in general these transgenically expressed
antibodies have properties that appear identical to those of antibodies expressed
in mouse cell cultures, from which the antibody to tetanus toxin was originally
derived. In addition, the single-chain antibody accumulates to about 1% of cellular
protein and appears to be quite stable within the chloroplast.
These studies indicate that pharmacologically important antibodies can be
produced in algae at high levels and that these antibodies act exactly as do
antibodies produced in more traditional and expensive systems. Chloroplast-produced
antibodies appear to be quite stable, a situation that should allow easy purification,
and algae-produced antibodies should lack many of the undesirable toxins that
often contaminate antibodies expressed in bacterial cells.
Chloroplasts are the site of synthesis of many key components of plants,
including lipids, amino acids, and carbohydrates. The main carbohydrate produced
by plant chloroplasts is starch, a storage compound produced at high levels that
humans use for a variety of purposes, including animal food and as a starting
material in many organic syntheses. A number of carbohydrates of pharmacologic
and commercial importance have starting material similar to that used in starch
We designed a gene construct to introduce a nonplant enzyme, haluronic acid
synthetase, into chloroplasts to determine if the novel carbohydrate hyaluronic
acid can be produced in C reinhardtii chloroplasts. Production of this
carbohydrate in microalgae could provide an important source for this scarce
compound, which is used in a variety of biomedical applications, including treatment
of arthritic joints.
Cohen, A., Yohn, C.B., Bruick, R., Mayfield, S.P. Translational regulation
of chloroplast gene expression in Chlamydomonas reinhardtii. Methods Enzymol.
Kim, J., Mayfield, S.P. Protein disulfide isomerase as a regulator
of chloroplast translational activation. Science 278:1954, 1997.
Mayfield, S.P., Cohen A. Translational regulation in the chloroplast.
Curr. Top. Plant Physiol. 19:174, 1998.
Yohn, C., Cohen, A., Danon, A., Mayfield, S.P. A poly(A) binding protein
functions in the chloroplast as a message specific translation factor. Proc.
Natl. Acad. Sci. U.S.A. 95:2238, 1998.