ARIA (for acetylcholine receptor-inducing activity), a protein purified on the basis of its ability to stimulate acetylcholine receptor (AChR) synthesis in cultured myotubes, is a member of the neuregulin family and is present at motor endplates. This suggests an important role for neuregulins in mediating the nerve-dependent accumulation of AChRs in the postsynaptic membrane. Nerve-muscle synapses have now been analyzed in neuregulin-deficient animals. Mice that are heterozygous for the deletion of neuregulin isoforms containing an immunoglobulin-like domain are myasthenic. Postsynaptic AChR density is significantly reduced, as judged by the decrease in the mean amplitude of spontaneous miniature endplate potentials and bungarotoxin binding. On the other hand, the mean amplitude of evoked endplate potentials was not decreased, due to an increase in the number of quanta released per impulse, a compensation that has been observed in other myasthenic states. Thus, the density of AChRs in the postsynaptic membrane depends on immunoglobulin-containing neuregulin isoforms throughout the life of the animal.

We have determined the structure of a metastable disulphide isomer of human insulin. Although not observed for proinsulin folding or insulin-chain recombination, the isomer retains ordered secondary structure and a compact hydrophobic core. Comparison with native insulin reveals a global rearrangement in the orientation of A- and B-chains. One face of the protein's surface is nevertheless in common between native and non-native structures. This face contains receptor-binding determinants, rationalizing the partial biological activity of the isomer. Structures of native and non-native disulphide isomers also define alternative three-dimensional templates. Threading of insulin-like sequences provide an experimental realization of the inverse protein-folding problem.

Transcriptional elongation provides a key control point in the regulation of eukaryotic gene expression. Here we describe homonuclear and 15N-heteronuclear 3D NMR studies of the nucleic acid binding domain of human transcriptional elongation factor TFIIS. This domain contains a Cys4 Zn(2+)-binding site with no homology to previously characterized Cys4, Cys6, or Cys2-His2 Zn fingers. Complete 1H and 15N NMR resonance assignment of a 50-residue TFIIS peptide-Zn2+ complex is obtained. Its solution structure, as determined by distance geometry/simulated annealing (DG/SA) calculations, exhibits a novel three-stranded antiparallel beta-sheet (designated the Zn ribbon). Analogous sequence motifs occur in a wide class of proteins involved in RNA or DNA transactions, including human basal transcriptional initiation factor TFIIE. A three-dimensional model of the TFIIE Cys4 domain is obtained by DG-based homology modeling. The role of the TFIIS Zn ribbon in the control of eukaryotic transcriptional elongation is discussed.