Three-dimensional structure topology of the calreticulin P-domain based onNMR assignment

Calreticulin (CRT) is an abundant molecular chaperone of the endoplasmic re ticulum. Its central, proline-rich P-domain, comprising residues 189-288, c ontains three copies of each of two repeat sequences (types 1 and 2), which are arranged in a characteristic '111222' pattern. Here we show that the t hree-dimensional structure of CRT(189-288) contains a single hairpin fold f ormed by the entire polypeptide chain. The loop at the bottom of the hairpi n consists of residues 227-247, and is closed by an anti-parallel beta -she et of residues 224-226 and 248-250. Two additional beta -sheets contain res idues 207-209 and 262-264, and 190-192 and 276-278, The 17-residue spacing of the beta -strands in the N-terminal part of the hairpin and the 14-resid ue spacing in the C-terminal part reflect the length of the type 1 and type 2 sequence repeats. As a consequence of this topology the peptide segments separating the beta -strands in the N-terminal part of the hairpin are lik ely to form bulges to accommodate the extra residues. These results are bas ed on nearly complete sequence-specific NMR assignments for CRT(189-288), w hich were obtained using standard NMR techniques with the C-13/N-15-labeled protein, and collection of nuclear Overhauser enhancement upper distance c onstraints. (C) 2001 Federation of European Biochemical Societies. Publishe d by Elsevier Science B.V. All rights reserved.