Oct. 9, 2009
Processes as diverse as receptor binding and signaling, cytoskeletal dynamics, and programmed cell death are manipulated by mimics of host proteins encoded by pathogenic bacteria. We show here that the Salmonella virulence factor SspH2 belongs to a growing class of bacterial effector proteins that harness and subvert the eukaryotic ubiquitination pathway. This virulence protein possesses ubiquitination activity that depends on a conserved cysteine residue. A crystal structure of SspH2 reveals a canonical leucine-rich repeat (LRR) domain that interacts with a unique E3 ligase [which we have termed NEL for Novel E3 Ligase] C-terminal fold unrelated to previously observed HECT or RING-finger E3 ligases. Moreover, the LRR domain sequesters the catalytic cysteine residue contained in the NEL domain, and we suggest a mechanism for activation of the ligase requiring a substantial conformational change to release the catalytic domain for function. We also show that the N-terminal domain targets SspH2 to the apical plasma membrane of polarized epithelial cells and propose a model whereby binding of the LRR to proteins at the target site releases the ligase domain for site-specific function.
Structure of the NEL domain of SspH2. (A) Comparison of the SspH2 NEL domain with 2 bacterial E3 ligases: SopA, a HECT family of cysteine-dependent E3 ubiquitin ligases from Salmonella (PDB ID 2QYU), and AvrPtoB, a RING finger/U-box protein (PDB ID 2FD4). The catalytic cysteine residues are shown in a space-filling format (blue). (B) Molecular surface representation of the crystallized construct of SspH2 with the LRR and NEL domains in orange and red, respectively. The catalytic cysteine is shown in blue. Hydrophobic residues in 2 clusters are show in green (LRR domain) and yellow (NEL domain, residues F570, F586, F587, V594, V597, M619, F620, L622, V633, and W645). (C) Residues near the catalytic cysteine of SspH2. Hydrogen bonds are shown as yellow spheres between atoms, and atoms of nitrogen and oxygen are shown in blue and red, respectively.
Results from: Quezada, C., Hicks, S., Galan, J., Stebbins, C. A Family of Salmonella Virulence Factors Functions as a Distinct Class of Autoregulated E3 Ubiquitin Ligases, Proc Natl Acad Sci USA, 106(12):4864-9, (2009).