Cysteine-based crosslinking approach for characterization of oligomeric pore-forming proteins in the mitochondrial membranes

Mitochondria are important not only to healthy but also dying cells. In particular, apoptotic cell death initiates when the mitochondrial outer membrane is permeabilized by Bax, a protein of the Bcl-2 family. Bax shares a structural fold with some a-helical bacterial pore-forming toxins before these proteins actively engage membranes. Despite decades of intensive research, the structures of the pores formed by these proteins are mostly unknown, mainly because the pores are assembled by different numbers of the proteins whose conformation and interaction are highly dynamic. Site-specific crosslinking of the pore-forming proteins in cellular membranes where the pores are assembled is a powerful approach to assess the biological pore structure, dynamics and function. In this chapter, we describe a cysteine-based site-specific crosslinking protocol for the Bax protein in the mitochondrial membrane. We discuss the expected results and the resulting structural-functional models for the pore-forming Bax oligomer, in comparison with other crosslinking approaches that have been used to study other mitochondrial protein complexes. At the end, we highlight the advantages of the crosslinking approaches as well as the limitations and alternative approaches.