Enhancin Protein Attacks Insect Immune System

June 25, 1997

ITHACA, N.Y. -- In the ongoing bout with plant pests, scientists at the Boyce Thompson Institute for Plant Research Inc., located at Cornell University, have found a one-two punch. The researchers have discovered and cloned a protein that, when delivered into an insect's gut by way of a "trojan horse," attacks the pest's intestines, rendering the pest helpless against a companion virus.

"No one has really studied the biochemistry and molecular biology of an insect's peritrophic matrix before," said Robert R. Granados, the Charles E. Palm Distinguished Scientist and virologist at Boyce Thompson Institute (BTI) and Cornell adjunct professor of entomology. "In our opinion, the peritrophic matrix (PM -- the interior lining of the insect's intestine,) is an important component in the insect's immune system against biological control agents. We've finally defined the site of the action of enhancin on the peritrophic matrix." Enhancin binds to a major mucinous protein of peritrophic matrix, destroying the matrix structure, thus allowing the baculovirus to pass through this relatively impermeable matrix unimpeded where it can kill the host, Granados explained.

Granados and colleagues reported their findings, "An intestinal mucin is the target substrate for a baculovirus enhancin," in Proceedings of the National Academy of Sciences (24 June 1997), authored by Granados and Ping Wang, Cornell graduate student in entomology. Another research report, "Molecular cloning and sequencing of a novel invertebrate intestinal mucin cDNA," is scheduled for publication in the Journal of Biological Chemistry (27 June 1997), authored by Wang and Granados.

The mucinous peritrophic matrix, the inside lining of the insect's intestine, is made mostly of chitin and proteins. It was previously believed that the insects' peritrophic matrix provided a barrier against certain microbial attack. What this discovery allows: the enhancin (an enzyme), when consumed by the insect, will break down the mucinous component of the peritrophic matrix, facilitating the passage of a virus, and resulting in the rapid death of the insect. "Mucins from mammals have been extensively studied," Granados said. "In contrast, knowledge about invertebrate mucins is very limited."

Wang and Granados discovered and cloned the first invertebrate intestinal mucin gene from the cabbage looper larvae (Trichoplusia ni). The researchers looked at seven types of pests and found their method to be effective against the fall armyworm, beet armyworm, the cotton bollworm, the American budworm, the velvetbean caterpillar and the common armyworm.

In the future, growers may use polyhedron crystals -- which contain enhancin and a pest-specific baculovirus. A pesky caterpillar, for example, will eat part of a leaf containing the polyhedron. The crystal would be dissolved in the insect's intestines, releasing the gut-wrenching enhancin and the baculovirus simultaneously. Neither the protein nor the baculovirus will harm humans or animals, Granados said. Acidity in human stomachs and intestines renders the protein ineffective against vertebrates. Invertebrate species -- such an insects -- do not have highly acidic intestinal contents.

The BTI researchers are seeking to genetically engineer the enhancin gene into plants, where it has considerable potential, Granados said.
-end-


Cornell University

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