Receptor binding and biological activity of mammalian expressed sensory and motor neuron-derived factor (SMDF)

Sensory and motor neuron-derived factor (SMDF) is a member of the neureguli n family of proteins, SMDF is structurally characterized by a novel N-termi nal domain. Using the signal sequence and N-terminal 28 amino acids (the "e pitope") of herpes simplex virus type 1 glycoprotein D (gD), we have expres sed SMDF as an epitope-tagged protein (gD-SMDF) in 293 cells, and purified it to > 98% homogeneity on a monoclonal anti-go column, gD-SMDF stimulates human Schwann cell growth and H-3-thymidine incorporation in MCF-7 and T47D human breast tumor cells in vitro. The biological activity of gD-SMDF is c onsistent with its ability to compete with I-125-labeled heregulin beta 1 p eptide (rHRG beta 1(177-244)) to bind to soluble dime;ic ErbB receptor-IgG fusion proteins. gD-SMDF binds with low affinity to homodimeric ErbB3-IgG a nd ErbB4-IgG but with higher affinity to heterodimeric ErbB2/ErbB3-IgG and ErbB2/ErbB4-IgG. Using a SMDF-IgG(Fc) fusion protein we generated a monoclo nal antibody (3G11) which binds SMDF, crossreacts with rHRG beta 1(177-244) , and neutralizes the in vitro activities of gD-SMDF and rHRG beta 1(177-24 4) in human Schwann cells.