Possible binding proteins of CP12 in a green alga,Chlamydomonas rein-hardtii, were investigated. We covalently immobilized CP12 on a resin and then used it to trap CP12 partners. Thus, we found an association between CP12 and phosphoribulokinase (EC ), glyceraldehyde 3-phosphate dehydrogenase (EC ) and aldolase.