Mapping the intermedilysin-human CD59 receptor interface reveals a deep correspondence with the binding site on CD59
Stephanie E Wickham1, Eileen M Hotze, Allison J Farrand
1Department of Microbiology and Immunology, the University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma 73104, USA.
Insights
Streptococcus intermedius intermedilysin (ILY) binds human CD59 (hCD59) to form pores. Despite binding common hCD59 sites, ILY and complement proteins C8α/C9 have distinct binding interfaces, impacting cytolytic activity.
Area of Science:
- Immunology
- Microbiology
- Structural Biology
Background:
- CD59 protein regulates complement membrane attack complex (MAC) formation.
- Streptococcus intermedius intermedilysin (ILY) is a cytolysin that binds hCD59 to initiate pore formation.
Purpose of the Study:
- To identify the binding interface between ILY and hCD59.
- To compare the hCD59 binding sites for ILY and MAC proteins.
- To understand the role of ILY-hCD59 interaction in cytolytic activity.
Main Methods:
- Residue identification of ILY and hCD59 binding interface.
- Analysis of hCD59 binding site homology with MAC proteins.
- Mutagenesis studies of hCD59 and ILY to assess cytolytic activity.
Main Results:
- A deep correspondence was found between hCD59 binding sites for ILY and MAC proteins C8α/C9.
- ILY disengages from hCD59 during the prepore to pore transition.
- Increased ILY-hCD59 affinity mutants showed decreased cytolytic activity.
- A novel hCD59-binding CDC family member was identified.
Conclusions:
- ILY and MAC proteins utilize common hCD59 amino acids but distinct binding interfaces.
- The dissociation of ILY from hCD59 is crucial for pore formation.
- ILY-hCD59 interaction dynamics influence cytolytic function.
Abstract:
CD59 is a glycosylphosphatidylinositol-anchored protein that inhibits the assembly of the terminal complement membrane attack complex (MAC) pore, whereas Streptococcus intermedius intermedilysin (ILY), a pore forming cholesterol-dependent cytolysin (CDC), specifically binds to human CD59 (hCD59) to initiate the formation of its pore. The identification of the residues of ILY and hCD59 that form their binding interface revealed a remarkably deep correspondence between the hCD59 binding site for ILY and that for the MAC proteins C8α and C9. ILY disengages from hCD59 during the prepore to pore transition, suggesting that loss of this interaction is necessary to accommodate specific structural changes associated with this transition. Consistent with this scenario, mutants of hCD59 or ILY that increased the affinity of this interaction decreased the cytolytic activity by slowing the transition of the prepore to pore but not the assembly of the prepore oligomer. A signature motif was also identified in the hCD59 binding CDCs that revealed a new hCD59-binding member of the CDC family. Although the binding site on hCD59 for ILY, C8α, and C9 exhibits significant homology, no similarity exists in their binding sites for hCD59. Hence, ILY and the MAC proteins interact with common amino acids of hCD59 but lack detectable conservation in their binding sites for hCD59.
