The beta-galactosidase from Escherichia coli was instrumental in the development of the operon model, and today is one of the most commonly 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 folds into five sequential domains, with an extended segment at the amino terminus. The participation of this amino-terminal segment in a subunit interface, coupled with the observation that each active site is made up of elements from two different subunits, provides a structural rationale for the phenomenon of alpha-complementation. The structure represents the longest polypeptide chain for which an atomic structure has been determined. Our results show that it is possible successfully to study non-viral protein crystals with unit cell dimensions in excess of 500 A and with relative molecular masses in the region of 2,000K per asymmetric unit. Non-crystallographic symmetry averaging proved to be a very powerful tool in the structure determination, as has been shown in other contexts.

In an attempt to simplify the protein folding problem, and also to further investigate the role of alanine as a helix-stabilizing residue, a series of alanines was introduced within the alpha-helix that includes residues 126-134 of T4 lysozyme. In wild-type lysozyme this alpha-helix contains alanine residues at positions 129, 130, and 134. Mutant lysozymes with alanines substituted at positions 128, 131, 132, and 133, either as single substitutions or in selected combinations, were constructed by oligonucleotide-directed mutagenesis. With the exception of the replacement of Leu 133, which is buried within the hydrophobic core of the protein, all the variants were more stable than wild-type lysozyme. The variant with alanines substituted at positions 128, 131, and 132 (E128A/V131A/N132A), which incorporates the sequence Ala 128-Ala 129-Ala 130-Ala 131-Ala 132-Leu 133-Ala 134, has a melting temperature 3.3 degrees C above that of wild-type lysozyme. Determination of the crystal structure of this mutant lysozyme shows that the replacement of Glu 128, Val 131, and Asn 132 with alanine causes alpha-helix 126-134 to rotate 3.4 degrees about an axis parallel to its own axis. This rotation seems to be triggered primarily by the loss of a hydrogen bond between Asn 132 and Ser 117 and is associated with the repacking of several side chains at the interface between alpha-helix 126-134 and the adjacent alpha-helix 115-122.(ABSTRACT TRUNCATED AT 250 WORDS)