Chaperone proteins help other proteins to fold. GroEL, the Escherichia coli form of the ubiquitous Cpn60 chaperonins, has a multimeric barrel-shaped structure with a central cavity, within which almost any protein can fold in vitro1. But what does GroE (GroEL plus its co-chaperone GroES) fold in the cell? Why is it needed for cell survival2? We report here the first definite identification of an essential, GroE-dependent E. coli protein, dihydropicolinate synthase (DapA), without which cell-wall synthesis fails.