In a cell free system , binding of a protein fragment ( 1762 1864 ) containing a pdz domain 4 from __NODE__ protein and a protein fragment ( aa sequence QSLV ) from __NODE__ protein is greater than binding of a protein fragment ( 1762 1864 ) containing a pdz domain 4 from __NODE__ protein and a protein fragment ( aa sequence QALV ) from mutant __NODE__ protein ( S?A ) .