This protein is the surface antigen or immobilization antigen of Paramecium primaurelia. Expression of G protein occurs at low temperatures (14-32 degrees Celsius). It has internal homologies and a highly periodic structure with 34 periods of about 75 residues, each period containing 8 cysteines, except for four half periods. A variable part of 475 residues comprises 4 almost identical periods in the middle of the protein.