The TGF-alpha precursor expressed on the cell surface binds to the EGF receptor on adjacent cells, leading to signal transduction.
The 50 amino acid form of TGF-alpha is cleaved from a conserved integral membrane glycoprotein by a protease that, in many tumor cells, appears to be limiting. To test whether the membrane-bound precursor has biological activity in the absence of processing, we introduced amino acid substitutions at the proteolytic cleavage sites. BHK cells transfected with expression vectors containing these altered sequences do not secrete detectable levels of mature TGF-alpha into the medium, but express high levels of proTGF-alpha at the cell surface. Coincubation of these BHK cells with A431 cells demonstrates that membrane-bound proTGF-alpha may bind to EGF receptors on the surface of contiguous cells, induce receptor autophosphorylation, and thereby produce a rapid rise in A431 intracellular calcium levels. Thus, proTGF-alpha can be biologically active in the absence of processing, a fact that may have implications for the integral membrane precursors of related growth factors.
Wong
S T
ST
Lineberger Cancer Research Center, University of North Carolina School of Medicine, Chapel Hill 27514.
Winchell
L F
LF
McCune
B K
BK
Earp
H S
HS
Teixidó
J
J
Massagué
J
J
Herman
B
B
Lee
D C
DC
D000595:Amino Acid Sequence; D000818:Animals; D015152:Blotting, Northern; D002450:Cell Communication; D002460:Cell Line; D002462:Cell Membrane / Q000502:physiology; D066246:ErbB Receptors / Q000502:physiology*; D007425:Intracellular Membranes / Q000378:metabolism; D008861:Microsomes / Q000378:metabolism; D008969:Molecular Sequence Data; D009154:Mutation; D010766:Phosphorylation; D019000:Phosphotyrosine; D010957:Plasmids; D011498:Protein Precursors / Q000235:genetics / Q000378:metabolism*; D015398:Signal Transduction; D014162:Transfection; D016211:Transforming Growth Factor alpha; D015291:Transforming Growth Factors / Q000235:genetics / Q000378:metabolism*; D014443:Tyrosine / Q000031:analogs & derivatives / Q000032:analysis
D011498:Protein Precursors; D016211:Transforming Growth Factor alpha; C052268:protransforming growth factor alpha; D019000:Phosphotyrosine; D014443:Tyrosine; D015291:Transforming Growth Factors; D066246:ErbB Receptors