SecB dependence of an exported protein is a continuum influenced by the characteristics of the signal peptide or early mature region.

We have used Escherichia coli alkaline phosphatase to show the interplay among the characteristics of two amino-terminal domains in the preprotein (the signal peptide and the early mature region), the efficiency with which this protein is transported, and its requirement for SecB to accomplish the transport process. The results suggest that although alkaline phosphatase does not normally require SecB for transport, it is inherently able to utilize SecB, and it does so when its ability to interface with the transport machinery is compromised.