A surface protein central to immune response homo-binds to other molecules like itself, according to a paper in the July 2005 issue of Nature Chemical Biology. The authors suggest that the activity of the glycoprotein CD22 is controlled by its binding to other CD22 molecules; and this could provide an understanding of the role played by the many examples of protein glycosylation in immune response signalling. Identifying the precise nature of these interactions will be pivotal to developing new treatments for immune system disorders.
Of central importance to the immune response, B cells are responsible for pathogen recognition and antibody production. Their cell surface contains many glycoproteins, or proteins with attached carbohydrates. These carbohydrates are often critical for mediating cell-signaling interactions, but identifying the proteins that bind to them and thus control the glycoproteins' function has always been difficult.
Using a clever approach involving in situ cross-linking of proteins and carbohydrates, James C. Paulson and colleagues were able to identify the binding partners of CD22 at the cell surface. CD22 modulation is mediated by glycoprotein carbohydrates terminating in sialic acid, an unusual 9-carbon sugar. The author's approach took advantage of the cell's own biosynthetic machinery to incorporate an analog of sialic acidcontaining a photoactive group into the carbohydrates of B-cell surface glycoproteins. Photoactivation then caused proximal cross-linking to proteins that bind to sialic acid. Their analysis of the resulting CD22-cross-linked complex suggested that CD22 is modulated by CD22 homo-binding and not by other molecules previously thought to be CD22 ligands.
Future applications of this approach may provide an understanding of the role played by the many examples of protein glycosylation in immune response signaling. Identifying the precise nature of these interactions will be pivotal to developing new treatments for immune system disorders.
James C. Paulson (The Scripps Research Institute, La Jolla, CA, USA)
Additional contact for comments on paper:
Kevin J. Yarema (The Johns Hopkins University, Baltimore, MD, USA)
Also published online(C) Nature Chemical Biology press release.
Message posted by: Trevor M. D'Souza