NOE, chemical shift and coupling constant data used in the structure calculation. All NOE's were assigned automatically using the CANDID module of CYANA. Cartoon representations of secondary structural elements are derived from the final family of structures. Coupling constants were extracted from a 15N-HMQC-J spectrum. Half-circles correspond to 3JNα < 4 Hz; filled circles correspond to 3JNα > 8 Hz, except in the case of residues 56-7, which have 3JNα values of 7.4 and 7.7 Hz respectively. These residues form part of strand β5, which is the least stable of the five strands making up the main β-sheet. The presence of a β-sheet comprised of strands β3 and β6, which does not occur in ubiquitin, is confirmed by large values of 3JNα, negative Cα shifts and αN(i,i+1) NOE's. The diagram was generated using CYANA .