3-DIMENSIONAL STRUCTURE OF BETA-GALACTOSIDASE FROM ESCHERICHIA-COLI

THE beta-galactosidase from Escherichia coli was instrumental in the d evelopment of the operon model(1), and today is one of the most common ly used enzymes in molecular biology. Here we report the structure of this protein and show that it is a tetramer with 222- point symmetry. The 1,023-amino-acid polypeptide chain(2,3) folds into five sequential domains, with an extended segment at the amino terminus. The particip ation of this amino-terminal segment in a subunit interface, coupled w ith the observation that each active site is made up of elements from two different subunits, provides a structural rationale for the phenom enon of alpha-complementation. The structure represents the longest po lypeptide chain for which an atomic structure has been determined. Our results show that it is possible successfully to study non-viral prot ein crystals with unit cell dimensions in excess of 500 Angstrom and w ith relative molecular masses in the region of 2,000K per asymmetric u nit. Non-crystallographic symmetry averaging proved to be a very power ful tool in the structure determination, as has been shown in other co ntexts(31,32).