In the pyruvate dehydrogenase multienzyme complex of Bacillus stearothermophilus, the interaction between the pyruvate decarboxylase (E1p) component and the lipoyl domain of the dihydrolipoyl acetyltransferase (E2) component was investigated using a combination of site-directed mutagenesis and NMR spectroscopy. Residues 11 to 15 (EGIHE) of the lipoyl domain, part of a surface loop close in space to the beta-turn containing the lipoyl-lysine residue (position 42), were deleted or replaced. The mutant domains all retained their three-dimensional structures and ability to become lipoylated, but in the absence of the loop the lipoyl-lysine residue could no longer be reductively acetylated by E1p. A mutation (N40A) in the N- terminal part of the lipoyl-lysine hairpin showed that it is involved in recognition of the domain by E1p but other mutations in the loop (E15A) and close to the lipoyl-lysine hairpin (V44S, V45S and E46A) were without effect. The heteronuclear multiple quantum coherence NMR spectra of 15N-labelled lipoyl domain in the presence and absence of B. stearothermophilus E1p were recorded. Of the 85 amino acid residues in the lipoyl domain, 13 exhibited significant differences in chemical shift. These differences, most of which were associated with residues in the surface loop between positions 8 and 15 and in, or close to, the lipoyl-lysine hairpin, indicate that E1p makes contact with the lipoyl domain in these areas. The combined results of directed mutagenesis and NMR spectroscopy point to the surface loop as a major determinant of the interaction of lipoyl domain with E1p. The specificity of this essential interaction provides the molecular basis of substrate channelling in this, the first committed, step of the enzyme reaction mechanism.