Escherichia coli chaperonin GroEL and GroES assist in folding of a wide variety of substrate proteins in the molecular mass range of 50 kDa, using cis mechanism, but limited information is available on how they assist in folding of larger proteins. Considering that the central cavity of GroEL can accommodate a non-native protein of 60 kDa, it is important to study the GroEL-GroES-assisted folding of substrate proteins that are large enough for cis encapsulation. In this study, we have reported the mechanism of GroEL/GroES-assisted in vivo and in vitro folding of a 69 kDa monomeric E. coli protein maltodextrin glucosidase (MalZ). Coexpression of GroEL and GroES in E. coli causes a 2-fold enhancement of exogenous MalZ activity in vivo. In vitro, GroEL and GroES in the presence of ATP give rise to a 7-fold enhancement in MalZ refolding. Neither GroEL nor single ring GroEL (SR1) in the presence or absence of ATP could enhance the in vitro folding of MalZ. GroES could not encapsulate GroEL- bound MalZ. All these experimental findings suggested that GroEL/GroES-assisted folding of MalZ followed trans mechanism, whereas denatured MalZ and GroES bound to the opposite rings of a GroEL molecule.--Paul, S., Singh, C., Mishra, S., Chaudhuri, T. K. The 69 kDa Escherichia coli maltodextrin glucosidase does not get encapsulated underneath GroES and folds through trans mechanism during GroEL/GroES-assisted folding.