The 1.28 A resolution crystal structure of acyl-CoA N-acyltransferase domain (ypeA) of putative acyltransferase from Escherichia coli str. K-12 substr. MG1655 was determined by molecular replacement. There are two similar in the tertiary structure (0.8 A r.ms.d. over 139 C-alpha atoms) copies of the protein within the I4 asymmetric unit that form a dimer (5800 A2). 38 hydrogen-bonded and 333 van der Waals contacts hold the dimer together. Crystal packing demonstrates that the domain may form octamers (32700 A2) too. The acyl-CoA N-acyltransferase domain is organized into a compact core subdomain with an antiparallel 4-stranded beta-sheet that is flanked by 4 helices. Additional strand-helix-strand structural motif represents a dimerization subdomain domain. There is an extended solvent-exposed groove between the dimerization and core domain that serves as a docking site for acyl-CoA molecule co-crystallized with the acyl-CoA N-acyltransferase domain (ypeA) (see Annotation for the PDB entry 4qvt).