This was a puzzle, because the antibody response is specific and limited to a single M protein type, while C4BP binds a broad variety of M protein types, perhaps up to 90 percent of them," said Ghosh. "Group A Strep brings C4BP to its surface to dampen the immune response. We wanted to combat this recruitment by blocking the interaction between M proteins and C4BP, but equally as importantly, we wanted to take advantage of the broad recruitment of C4BP by M proteins that would pave a path to the development of a vaccine."
To determine if this was possible, a graduate student in Ghosh's lab, Cosmo Buffalo, collaborated with another graduate student, Sophia Hirakis, in the laboratory of Rommie Amaro, a professor of chemistry and biochemistry who uses computers to study protein structures, to first study the complex interactions between M protein and C4BP.
"This allowed us to understand some detailed features of the interaction," said Ghosh. The research team, which also included an undergraduate researcher, Adrian Bahn-Suh, collaborated extensively with Victor Nizet, an expert in infectious diseases who is a professor at UC San Diego's School of Medicine and Skaggs School of Pharmacy and Pharmaceutical Sciences.
In their experimental and computational study, the biochemists painstakingly detailed four crystal structures of four different M protein types, each bound to human C4BP.
"These structures revealed that even though the different M protein types appeared to be unrelated in sequence, there were common sequence patterns hidden within the differences that linked all these M proteins together," said Ghosh. "These common patterns are what is used to recruit C4BP to the surface of group A Strep by the different M protein types."
"The idea now is to have antibodies do the same thing as C4BP -- that is, recognize many different M protein types," he added. "That way, the antibody response will not be limited to one M protein type and one strain of group A Strep, but will extend to most, if not all, M protein types and most, if not all strains, of group A Strep."
The UC San Diego chemists, in collaboration with Nizet, are now working on developing a vaccine that, they hope, will be protective against most, if not all, strains of group A Strep.
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