Unconventional secretion of fibroblast growth factor 2 is mediated by direct translocation across the plasma membrane of mammalian cells.
Fibroblast growth factor 2 (FGF-2) is a pro-angiogenic mediator that is secreted by both normal and neoplastic cells. Intriguingly, FGF-2 has been shown to be exported by an endoplasmic reticulum/Golgi-independent pathway; however, the molecular machinery mediating this process has remained elusive. Here we introduce a novel in vitro system that functionally reconstitutes FGF-2 secretion. Based on affinity-purified plasma membrane inside-out vesicles, we demonstrate post-translational membrane translocation of FGF-2 as shown by protease protection experiments. This process is blocked at low temperature but apparently does not appear to be driven by ATP hydrolysis. FGF-2 membrane translocation occurs in a unidirectional fashion requiring both integral and peripheral membrane proteins. These findings provide direct evidence that FGF-2 secretion is based on its direct translocation across the plasma membrane of mammalian cells. When galectin-1 and macrophage migration inhibitory factor, other proteins exported by unconventional means, were analyzed for translocation into plasma membrane inside-out vesicles, galectin-1 was found to be transported as efficiently as FGF-2. By contrast, migration inhibitory factor failed to traverse the membrane of inside-out vesicles. These findings establish the existence of multiple distinct secretory routes that are operational in the absence of a functional endoplasmic reticulum/Golgi system.
Schäfer
Tobias
T
Heidelberg University Biochemistry Center, Im Neuenheimer Feld 328, 69120 Heidelberg, Germany.
Zentgraf
Hanswalter
H
Zehe
Christoph
C
Brügger
Britta
B
Bernhagen
Jürgen
J
Nickel
Walter
W
D000255:Adenosine Triphosphate / Q000737:chemistry / Q000378:metabolism; D000818:Animals; D016827:CD8 Antigens / Q000096:biosynthesis; D016466:CHO Cells; D002462:Cell Membrane / Q000378:metabolism* / Q000648:ultrastructure; D006224:Cricetinae; D003600:Cytosol / Q000378:metabolism; D003902:Detergents / Q000494:pharmacology; D004721:Endoplasmic Reticulum / Q000378:metabolism; D016222:Fibroblast Growth Factor 2 / Q000378:metabolism*; D037483:Galectin 1 / Q000378:metabolism; D006056:Golgi Apparatus / Q000378:metabolism; D006868:Hydrolysis; D008263:Macrophage Migration-Inhibitory Factors / Q000378:metabolism; D008854:Microscopy, Electron; D008954:Models, Biological; D021381:Protein Transport; D012967:Sodium Dodecyl Sulfate / Q000737:chemistry; D013696:Temperature; D013997:Time Factors
D016827:CD8 Antigens; D003902:Detergents; D037483:Galectin 1; D008263:Macrophage Migration-Inhibitory Factors; D016222:Fibroblast Growth Factor 2; D012967:Sodium Dodecyl Sulfate; D000255:Adenosine Triphosphate