Protein-tyrosine kinases regulate the phosphorylation, protein interactions, subcellular distribution, and activity...

Area of Science:

• Cellular Biology
• Molecular Biology
• Biochemistry

Background:

• GTPase-activating protein (GAP) is crucial for down-regulating p21ras activity by stimulating GTPase activity.
• In normal cells, GAP primarily exists as a monomer in the cytosol.
• Activation of protein-tyrosine kinases or growth factor stimulation alters GAP's cellular localization and complex formation.

Purpose of the Study:

• To investigate how activated protein-tyrosine kinases and epidermal growth factor (EGF) stimulation affect the complex formation and activity of GAP.
• To identify the specific phosphoproteins that interact with GAP under these conditions.
• To elucidate the role of these complexes in p21ras signaling pathways.

Main Methods:

• Western blotting and immunoprecipitation to detect GAP and its associated proteins.
• Analysis of GAP phosphorylation status on tyrosine and serine residues.
• Cellular fractionation and localization studies.
• Assays to measure GAP's GTPase-activating protein activity.

Main Results:

• GAP undergoes tyrosine and serine phosphorylation upon activation of p60v-src or EGF stimulation.
• Phosphorylated GAP forms distinct complexes with p62 and p190 phosphoproteins.
• The GAP-p62 complex is found in the plasma membrane and cytosol, while the GAP-p190 complex is exclusively cytosolic and shows reduced GAP activity.
• EGF stimulation leads to the conversion of monomeric GAP to higher-molecular-weight species.

Conclusions:

• Protein-tyrosine kinases induce the formation of multiple heteromeric GAP complexes.
• These complexes, particularly the GAP-p190 complex, are likely involved in the regulation or targeting of p21ras.
• The phosphorylation-dependent complex formation alters GAP activity, suggesting a regulatory mechanism for p21ras signaling.