Trends in Biochemical Sciences
Disulfide bonds as switches for protein function
Section snippets
Disulfide cleavage in secreted soluble proteins
One of the first examples of disulfide cleavage described in a secreted protein was in thrombospondin (TSP)-1. TSP-1 is an extracellular glycoprotein that participates in cell–cell and cell–matrix communication, and plays a role in the growth and differentiation of tissues. The TSP family consists of five members in vertebrates [3]. TSP-1 is a homotrimer of disulfide-linked 150-kDa monomers [4] and each subunit contains a free thiol [5]; in Ca2+-depleted TSP-1, this thiol can reside on any one
Disulfide cleavage in cell-surface receptors
Disulfide bonds in the extracellular domains of some cell-surface receptors are also cleaved. The best-characterized examples to date are the immune-cell receptor CD4, the HIV-1 envelope glycoprotein gp120 and the integrin receptor αIIbβ3.
CD4 is a member of the immunoglobulin (Ig) superfamily of receptors that mediates cell–cell interactions in the immune system and is the primary receptor for HIV-1. HIV-1 binds to CD4 via its gp120 envelope protein. This binding leads to interaction of the
How disulfide bonds are cleaved outside the cell
There is more than one way that disulfide bonds can be cleaved in the extracellular environment: three possible mechanisms of disulfide-bond cleavage have been identified or suggested in secreted proteins (Fig. 1).
Prediction of disulfide cleavage in proteins?
As more examples of disulfide cleavage in proteins are uncovered and the mechanism of cleavage is elucidated, it will be possible to begin making predictions about similar events in other proteins. With this in mind, my colleagues and I wondered whether the unusual features of the disulfide bond in CD4 D2 are mirrored in other proteins.
Cleavage of the CD4 D2 disulfide bond was suggested by the unusual geometry and strain of the bond [34]. The backbone of Ig domains are defined by two β sheets,
Concluding remarks
The evidence indicates that disulfide bonds have been added to proteins not only to help hold them together, but also as a way of controlling how they work. Identification of the common molecular events that lead to cleavage of disulfide bonds in extracellular proteins, whether the cleavage is reversible and how prevalent this form of protein regulation is in biology, are all pressing issues.
The dithiol–disulfide redox exchange mechanism for cleaving disulfide bonds in the extracellular milieu
Acknowledgements
I thank Merridee Wouters for many helpful discussions about cross-strand disulfides and Neil Donoghue for his help with describing the mechanisms of disulfide-bond cleavage. This work was supported by the National Health and Medical Research Council of Australia, the Australian Research Council, and the New South Wales Cancer Council and Health Department.
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