J. Biol. Chem., Vol. 261, Issue 15, 6643-6646, May, 1986
Internal structural features of E. coli glycyl-tRNA synthetase examined by subunit polypeptide chain fusions
MJ Toth and P Schimmel
In contrast to most aminoacyl-tRNA synthetases which are monomers or
oligomers of a single polypeptide, Escherichia coli glycyl-tRNA synthetase
has an alpha-2, beta-2 structure. The enzyme requires both subunits for
catalysis of either adenylate or aminoacyl-tRNA synthesis. The head-to-tail
arrangement of the alpha- and beta-chain coding regions in the genome
suggests that the two-subunit protein may be tantamount to a single chain.
We fused the carboxyl terminus of the alpha-chain to the amino terminus of
the beta-chain, through a short peptide linker. Five different amino acid
substitutions were placed in the linker. In all instances, the fusion
polypeptide is stable in maxicell extracts. In a glyS null strain, a gene
encoding any of the fusion proteins substitutes for the wild-type gene.
Assays confirm that, in vitro, the engineered polypeptide fusion is active
to within 2- to 3-fold of the wild-type, unfused chains. Oligomers of the
fusion protein are observed and may be required for activity. Because the
creation and limited manipulation of the artificial peptide linker region
does not destroy the activity, we also conclude that the C- terminal part
of the alpha-chain and the amino-terminal part of the beta-chain are not
important for activity.