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Transmembrane transforming growth factor-alpha tethers to the PDZ domain-containing, Golgi membrane-associated
1Departments of Growth and Development, and Anatomy, Programs in Cell Biology and Developmental Biology, University of California at San Francisco, San Francisco, CA 94143-0640, USA.
Abstract:
Transforming growth factor-alpha (TGF-alpha) and related proteins represent a family of transmembrane growth factors with representatives in flies and worms. Little is known about the transport of TGF-alpha and other transmembrane growth factors to the cell surface and its regulation. p59 was purified as a cytoplasmic protein, which at endogenous levels associates with transmembrane TGF-alpha. cDNA cloning of p59 revealed a 452 amino acid sequence with two PDZ domains. p59 is myristoylated and palmitoylated, and associates with the Golgi system, where it co-localizes with TGF-alpha. Its first PDZ domain interacts with the C-terminus of transmembrane TGF-alpha and select transmembrane proteins. p59 is the human homolog of GRASP55, which is structurally related to GRASP65. GRASP55 and GRASP65 have been shown to play a role in stacking of the Golgi cisternae in vitro. C-terminal mutations of transmembrane TGF-alpha, which decrease or abolish the interaction with p59, also strongly impair cell surface expression of TGF-alpha. Our observations suggest a role for membrane tethering of p59/GRASP55 to select transmembrane proteins, including TGF-alpha, in maturation and transport to the cell surface.
Insights
p59 (GRASP55) protein binds to transforming growth factor-alpha (TGF-alpha), aiding its transport to the cell surface. This discovery sheds light on the regulation of transmembrane growth factor trafficking.
Area of Science:
- Cell Biology
- Molecular Biology
- Protein Trafficking
Background:
- Transforming growth factor-alpha (TGF-alpha) is a transmembrane growth factor with poorly understood cell surface transport mechanisms.
- p59 is a newly identified cytoplasmic protein that associates with TGF-alpha.
Purpose of the Study:
- To investigate the role of p59 in the maturation and cell surface transport of TGF-alpha.
- To characterize the interaction between p59 and TGF-alpha.
Main Methods:
- Protein purification and association studies.
- cDNA cloning and sequence analysis of p59.
- Immunofluorescence microscopy to assess co-localization.
- Site-directed mutagenesis to study protein interactions.
Main Results:
- p59, the human homolog of GRASP55, was identified and cloned, revealing two PDZ domains and post-translational modifications (myristoylation, palmitoylation).
- p59 co-localizes with TGF-alpha in the Golgi system, with its PDZ domain interacting with TGF-alpha's C-terminus.
- Mutations disrupting p59-TGF-alpha interaction significantly impair TGF-alpha cell surface expression.
Conclusions:
- p59/GRASP55 plays a crucial role in the maturation and cell surface transport of TGF-alpha.
- Membrane tethering of p59/GRASP55 to transmembrane proteins like TGF-alpha is essential for proper trafficking.