Measurement clarifies role between protein structure and cell adhesion

Scientists studying the adhesive properties of the neural cell adhesion molecule (NCAM) – a protein that helps bind the nervous system together – have found that two opposing models of cell adhesion are both correct.

“Our extremely sensitive technique allows us to directly measure how these proteins bind to one another, and to further explore the relationship between their structure and function,” said Deborah Leckband, a professor and head of chemical and biomolecular engineering at the University of Illinois at Urbana-Champaign and corresponding author of a paper to be published the week of April 26 in the Online Early Edition of the Proceedings of the National Academy of Sciences.

Important in neural development and for linking muscles to neurons, NCAM is a membrane-anchored protein that holds cells together through bonds formed between five modular regions called domains. Previous studies had generated two seemingly contradictory models for NCAM adhesion that involved different domains.

To directly study the adhesive properties of NCAM, Leckband and her colleagues used a surface-force apparatus to measure the molecular forces between two NCAM monolayers as a function of the distance between them.

“Our direct-force measurements show that NCAM binds in two spatially distinct configurations that result in different membrane separations,” said Leckband, who also is a researcher at the university’s Beckman Institute for Advanced Science and Technology. “The protein’s modular architecture permits the formation of multiple bonds that engage different modules, which allowed us to directly test both models.”

Previous studies of NCAM binding could detect only one or the other configuration, Leckband said, thereby creating an apparent contradiction between two opposing models. The researchers’ measurements confirm both models, but disprove a recently proposed third model that was based upon a recently published crystal structure.

“Many research groups rely on crystal structures to determine the nature of chemical interactions that occur between the molecules when they are bound,” Leckband said. “But we are finding that, particularly for these weakly binding interactions, there are other factors that influence how the crystal is formed that override the physical interactions.”

By showing that NCAM forms either of two adhesive configurations, which require different domains and span different membrane separations, the researchers have reconciled several apparently contradictory experimental results, and validated two of the current models as contributing to spatially and molecularly distinct NCAM bonds.

“The different bonding configurations may serve as scaffolds that hold the membranes apart and regulate the intercellular space,” Leckband said. “The scaffolds would allow some molecules in – like some proteins that activate the immune response – while excluding others.”

Co-authors of the paper were graduate student Colin Johnson at the U. of I., and Ichiro Fujimoto, Claire Perrin-Tricaud and Urs Rutishauser at the Memorial Sloan-Kettering Cancer Center in New York City. The National Institutes of Health funded the work.