The J-domain is a highly conserved domain found in all members of the DnaJ family of molecular chaperones. The three-dimensional structure of a recombinant, uniformly 15N-labeled 77-residue polypeptide containing the complete J-domain from human Hsp40 (HDJ-1) has been determined by nuclear magnetic resonance (NMR) spectroscopy in solution. On the basis of 876 upper distance constraints derived from nuclear Overhauser effects (NOE) and 173 dihedral angle constraints, a group of 20 conformers representing the solution structure of the HDJ-1 J-domain was computed with the program DIANA and energy-minimized with the program OPAL. The average of the pairwise root-mean-square deviations of the individual NMR conformers relative to the mean coordinates for the backbone atoms N, C2 and C' of residues 4 to 54 and 4 to to 66 is 0.88 and 0.99 A respectively. The molecular architecture includes four helices composed of residues 5 to 9, 15 to 28, 40 to 54 and 60 to 66. A turn composed of residues 10 to 14 links helices I and II, and a loop composed of residues 29 to 39 containing a highly conserved tripeptide HPD (residues 31 to 33) connects the antiparallel helices II and III. The tertiary fold formed by helix I-turn-helix II-loop-helix III forms a closed structural core; the less defined helix IV stands away from the core of the domain. The side-chains of the tripeptide HPD extend out from the core of the structure in the opposite direction from helix IV. The structure supports the hypothesis that the highly conserved tripeptide could play a key role in the interaction of Hsp40 with the molecular chaperone, Hsp70.