BiochemistryA mutant cholera toxin B subunit that binds GM1- gangliosidebut lacks immunomodulatory or toxic activity
(2001)
Journal - PNAS
Abstract :
* Department of Pathology and Microbiology, University of Bristol,
Bristol BS81TD, United Kingdom; Department of Structural
Biology and ¶ Howard Hughes Medical Institute, University
of Washington, Seattle, WA 98195; and § Combined Program in
Pediatric Gastroenterology and Nutrition, Children's Hospital,
Boston, MA 02115
Communicated by Linda L. Randall, University of Missouri,
Columbia, MO, May 31, 2001 (received for review February 6, 2001)
GM1-ganglioside receptor binding by the B subunit of cholera toxin
(CtxB) is widely accepted to initiate toxin action by triggeringuptake
and delivery of the toxin A subunit into cells. More recently,GM1
binding by isolated CtxB, or the related B subunit of
Escherichiacoli heat-labile enterotoxin (EtxB), has
been found to modulateleukocyte function, resulting in the
down-regulation of proinflammatoryimmune responses that cause
autoimmune disorders such as rheumatoidarthritis and diabetes. Here,
we demonstrate that GM1 binding,contrary to expectation, is not
sufficient to initiate toxin action.We report the engineering and
crystallographic structure of amutant cholera toxin, with a His to Ala
substitution in the Bsubunit at position 57. Whereas the mutant
retained pentamericstability and high affinity binding to
GM1-ganglioside, it hadlost its immunomodulatory activity and, when
part of the holotoxincomplex, exhibited ablated toxicity. The
implications of thesefindings on the mode of action of cholera toxin
arediscussed.