The amino acid sequence of ERp57, which functions in the endoplasmic reticulum together with the lectins calreticulin and calnexin to achieve folding of newly synthesized glycoproteins, is highly similar to that of protein disulfide isomerase (PDI), but they have their own distinct roles in protein folding. We have characterized the domain structure of ERp57 by limited proteolysis and N-terminal sequencing and have found it to be similar but not identical to that of PDI. ERp57 had three major protease-sensitive regions, the first of which was located between residues 120 and 150, the second between 201 and 215, and the third between 313 and 341, the data thus being consistent with a four-domain structure abba. Recombinant expression in Escherichia coli was used to verify the domain boundaries. Each single domain and a ba double domain could be produced in the form of soluble, folded polypeptides, as verified by circular dichroism spectra and urea gradient gel electrophoresis. When the ability of ERp57 and its a and a domains to fold denatured RNase A was studied by electrospray mass analyses, ERp57 markedly enhanced the folding rate at early time points, although less effectively than PDI, but was an ineffective catalyst of the overall process. The a and a domains produced only minor, if any, increases in the folding rate at the early stages and no increase at the late stages. Interaction of the soluble ERp57 domains with the P domain of calreticulin was studied by chemical cross-linking in vitro. None of the single ERp57 domains nor the ba double domain could be cross-linked to the P domain, whereas cross-linking was obtained with a hybrid ERpabbPDIac polypeptide but not with ERpabPDIbac, indicating that multiple domains are involved in this protein-protein interaction and that the b domain of ERp57 cannot be replaced by that of PDI.ERp57 is a protein resident in the endoplasmic reticulum (ER) 1 that functions as a co-chaperone in glycoprotein folding with the lectins calnexin (CNX) and its soluble homologue calreticulin (CRT) (for reviews, see Refs. 1 and 2). ERp57 is a member of the protein disulfide isomerase (PDI) oxidoreductase family (3, 4) and has two thioredoxin-like domains with -Cys-Gly-His-Cys-catalytic site motifs in positions corresponding to the thioredoxin-like domains a and aЈ of PDI, which consists altogether of domains a, b, bЈ, and aЈ plus an acidic C-terminal extension c (5). ERp57 also shows a significant sequence similarity to PDI in regions corresponding to domains b and bЈ (6). No similarity is found in the organization of the human genes or gene loci for ERp57 and PDI, however (7). Despite the 56% overall amino acid sequence similarity between PDI and ERp57, the latter does not substitute for PDI as the ␤ subunit in the assembly of the collagen prolyl 4-hydroxylase ␣ 2 ␤ 2 tetramer (6), an enzyme involved in the synthesis of collagens (8 -10), but the N-terminal domains a and b of ERp57 can in part substitute for those of PDI in this assembly (11). ERp57 possesses disulfide oxidoreduc...

a b s t r a c tRemF is a polyketide cyclase involved in the biosynthesis of the aromatic pentacyclic metabolite resistomycin in Streptomyces resistomycificus. The enzyme is a member of a structurally hitherto uncharacterized class of polyketide cyclases. The crystal structure of RemF was determined by SAD and refined to 1.2 Å resolution. The enzyme subunit shows a b-sandwich structure with a topology characteristic for the cupin fold. RemF contains a metal binding site located at the bottom of the predominantly hydrophobic active site cavity. A zinc ion is coordinated to four histidine side chains, and two water molecules in octahedral ligand sphere geometry, highly unusual for zinc binding sites in proteins.

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To our knowledge, this is the first report of early-onset isolated foot metatarsal arthritis with apparent autosomal dominant inheritance.

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