The availability of cloned cDNAs encoding the four subunits of the Torpedo acetylcholine receptor, which can be expressed to make functional receptors in Xenopus oocytes, has made possible a detailed investigation of the functions of the different structural components of the receptor. The functional analysis of receptors with alpha-subunits altered at specific sites by site-directed mutagenesis of the cDNA has allowed the location of specific regions of the alpha-subunit molecule involved in acetylcholine binding and forming a transmembrane ionic channel.

The combination of complementary DNA expression and single-channel current analysis provides a powerful tool for studying the structure-function relationship of the nicotinic acetylcholine receptor (AChR) (refs 1-5). We have previously shown that AChR channels consisting of subunits from different species, expressed in the surface membrane of Xenopus oocytes, can be used to relate functional properties to individual subunits. Here we report that, in extracellular solution of low divalent cation concentration, the bovine AChR channel has a smaller conductance than the Torpedo AChR channel. Replacement of the delta-subunit of the Torpedo AChR by the bovine delta-subunit makes the channel conductance similar to that of the bovine AChR channel. To locate the region in the delta-subunit responsible for this difference, we have constructed chimaeric delta-subunit cDNAs with different combinations of the Torpedo and bovine counterparts. The conductances of AChR channels containing chimaeric delta-subunits suggest that a region comprising the putative transmembrane segment M2 and the adjacent bend portion between segments M2 and M3 is involved in determining the rate of ion transport through the open channel.

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The Torpedo and calf acetylcholine receptors and hybrids composed of subunits from the two species have been produced in Xenopus oocytes by the use of the cloned complementary DNAs. Single-channel current measurements indicate that these receptors form channels of similar conductance but with different gating behaviour.

Metallothioneins (MT) are metalloproteins expressed tissue specifically during the development of the sea urchin, Strongylocentrotus pururutus. To explore their structural and functional features and to compare them with those of the evolutionary distant mammalian MTs, one isoform (MTA) was obtained as the cadmium-containing form, from synthetic cDNA heterologously expressed in Escherichia coli. The purified protein was identified as the desired product by a combination of peptide-map analysis, amino acid sequence analysis and ion-spray mass spectroscopy. The existence of seven " T d NMR resonances revealed that the recombinant protein binds seven Cd ions/molecule. The position of the NMR resonances (605 -695 ppm) and the electronic absorption features suggest that the sea urchin MTA, like the mammalian MTs, possesses tetrahedrally coordinated cadmiumthiolate clusters. With its large Stokes' radius, sea urchin MTA resembles the mammalian forms, suggesting a comparable elongated molecular shape. Measurements by spectrophotometric pH titration of cadmium binding by the recombinant protein suggest that it possesses two metal-thiolate clusters of distinctly different stability. At pH 7 the average apparent association constant for Cd2' in the clusters is about 20-times weaker in sea urchin MTA than in rabbit MT-2.Metallothioneins (MT) are widely found low-molecularmass, cysteine-rich proteins distinguished by an exceptionally high content of d' O metal ions (Zn", Cd2+ and Cu'+) (see Roesijadi, 1993, andKagi, 1993, for reviews). MTs have traditionally been thought to be involved in cellular detoxification of metals but more central functions such as an involvement in the regulation of Zn-dependent processes in gene expression (Zeng et al., 1991a and and in development (Nemer et al. 1984;De et al., 1991) have also been suggested.The sea urchin Strongylocentotus pupurutus like many other species, contains a family of MT genes. Two of these genes, designated MTA and MTB,, have been well characterised. These genes show a high degree of identity, but probes specific for their mRNAs have been constructed and used to show that the expression patterns of the genes are quite distinct. Both mRNAs begin to accumulate at the early blastula stage (12 h after fertilization) but the MTB, mRNA becomes localized in the embryonic gut and in the oral ectoderm while the MTA mRNA is spatially restricted to the aboral ectoderm (Nemer et al., 1991). As found for mammalian MTs, both forms are readily inducible by zinc although with unequal thresholds (Wilkinson and Nemer, 1987). This responsiveness to zinc supply is accounted for by the occurrence of at least six metal-regulatory elements (MRE) in the 5' upstream region of both the sea urchin genes (Bai et al., 1993). The two MREs of the MTA gene (designated a and b) nearest to the transcription start site are identical in both sequence and position (relative to the TATA box) to those which give maximal activity in mammalian systems (Harlow et al., 1989). In addition to this conservation of...