But somehow, a resourceful family of viruses known as rhinoviruses has developed a back-door way to use this receptor to enter human cells.
The Purdue study shows that the rhinovirus bypasses the structure that ICAM-1 uses to bind to white blood cells, and binds instead to another part of the receptor to gain entry into the cell, says Jordi Bella, a postdoctoral researcher working with Rossmann on the study.
"Our study shows that the very tip of the ICAM-1 molecule is shaped somewhat like a hand, with a thumb and three projections, or fingers," Bella says. "Normally, white blood cells bind to the thumb-like projection. But the virus binds to the three finger-like projections, and interacts with the receptor to gain entry into the cell."
These finger-like projections are what sets ICAM-1 apart from other cellular adhesion molecules, and they make it a perfect complement to the rhinovirus structure, says Rossmann, who in 1986 became the first scientist to solve the structure of a cold virus.
The finger-like projections also may distinguish human ICAM-1 from the ICAM-1 found in all other animals, except chimpanzees, and may explain why only humans and chimpanzees are infected by the cold virus.
"The shell of the rhinovirus has deep crevices or canyons capable of interacting with the finger-like projections of the ICAM-1 receptor," Rossmann says. "The virus probably has adapted itself to be able to attach to this particular molecule in humans, so that they fit exactly, similar to a lock and key."
As the virus attaches to one or more receptor sites on a cell, the cell membrane engulfs the virus, wrapping around it and allowing it to come in contact with more receptor sites. The Purdue group now is pursuing studies to determine how this interaction allows the shell of the rhinovirus to unfold and release its genetic material into the cell.
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Contact: Susan Gaidos
susan_gaidos@uns.purdue.edu
765-494-2081
Purdue University
14-Apr-1998