Phosphoglucose isomerase (PGI) is a cytosolic glycolytic enzyme that also functions as an extracellular cytokine (neuroleukin/autocrine motility factor (AMF)/maturation factor). Contrary to mammalian PGI, bacterial PGI was not internalized by the PGI/AMF receptor (gp78/AMF-R) and neither bacterial nor yeast PGI competed with mammalian PGI for receptor binding and internalization. Furthermore, while the bacterial, yeast and mammalian preparations all exhibited isomerase activity, only mammalian PGI stimulated the motility of NIH-3T3 fibroblasts. The conserved active site of PGI is therefore not sufficient for receptor binding and cytokine activity of PGI. However, synthetic peptides corresponding to distinct peripheral mammalian PGI sequences did not inhibit internalization of mammalian PGI. Our data therefore argue that the cytokine activity of PGI is specific to mammalian PGI but cannot exclude the possibility that the receptor binding motif of PGI is complex and includes elements within and without the active site.