Staphylococcal protein A, Z Domain

Staphylococcal protein A, or SpA, is a type I membrane protein from the bacterium Staphylococcus aureus. It is bound to the cell wall via its C-terminal cell-wall-binding region X. In the N-terminal half of the protein is its IgG-binding domains E, D, A, B, and C. SpA-IgG interactions are important because it may help to unlock details about alpha-helical proteins and their IgG-like receptors such as cytokines and growth factors.
The Z domain of SpA is an engineered analogue if the IgG-binding domain B. Z domain contains three alpha helices which are arranged in an antiparallel three-helix bundle.
In the picture shown above, the Z domain of SpA is colored by chain. Four types of side chains are visible however: lysine (K)=blue, phenylalanine (F)=aqua, leucine (L)=green, and aspartic acid (D)=red. This domain of the protein contains no disulfide bonds and is 72 amino acid residues long.

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