Streptococcal protein G. Computer model showing the primary (ball-and-stick) and secondary (alpha- helix, blue, and beta-sheets, purple) structures of Streptococcal protein G. This is a cell wall protein from Streptococcus sp. bacteria. It binds the human antibody immunoglobulin G (IgG), part of the body's immune system. However, this is not the protein's primary function, which, although unknown, is thought to be connected with essential bacterial functions.