Pendant of a Protein A domain in 1.2 mm silver wire. The protein thread is colored in the colors of the rainbow, from blue (first amino acid) to red (last amino acid).

Protein A is a protein found in the cell wall of the bacteria Staphylicoccus areus. The protein is composed of five homologous domains that fold into a three-helix bundle. Each domain is able to bind immunoglobulins.