Assembling the building blocks: structure and function of inhibitor of apoptosis proteins.

Control of apoptotic signalling pathways depends on the balance between proapoptotic and prosurvival molecules. The 'inhibitor of apoptosis' (IAP) proteins are negative regulators of apoptosis that function by inhibiting the executioners of cell death (caspases), or by blocking the pathways that activate them. The IAP proteins function as ubiquitin E3 ligases and possess protein-protein interaction domains. IAPs can promote the addition of ubiquitin to themselves and to the substrate proteins that they interact with either directly or indirectly through adaptor proteins. The balance between substrate and autoubiquitylation seems to be important for their activity. In this review, we describe the structural features of IAP proteins as they are currently understood, and discuss how each domain contributes to IAP function. It is clear that to advance our understanding of these complex proteins, we must decipher how the domains operate in concert and how each domain impacts on the activity of the other.