Precursors of three exported proteins in Escherichia coli

Arabinose-binding protein, maltose-binding protein, and lambda receptor are synthesized in vitro on membrane-bound polysomes from Escherichia coli. All three proteins are exported from the cytoplasm of E. coli and all three are made in vitro in a form a few thousand daltons larger than the authentic protein. The larger form of arabinose-binding protein is also detected in vivo by pulse labeling. It is concluded that the larger forms of the exported proteins are precursors containing an extra sequence. In contrast to the above, when the intracellular protein elongation factor Tu is synthesized in vitro on free polysomes, it is not detectably larger than the authentic form.