The cycle of SEC4 function in vesicular transport

Area of Science:

• Cell Biology
• Molecular Biology
• Biochemistry

Background:

• Sec4 is a Ras-like GTP-binding protein essential for yeast exocytosis.
• Sec4's function depends on cycling between GTP- and GDP-bound states, not just levels of the GTP-bound form.
• This cycling is linked to Sec4's intracellular localization, including binding to secretory vesicles and recycling from the plasma membrane.

Purpose of the Study:

• To investigate the regulatory mechanisms controlling Sec4's GTP/GDP binding cycle.
• To identify proteins involved in releasing GDP-bound Sec4 from membranes.
• To understand how Sec4's cycle facilitates the assembly/disassembly of exocytosis machinery.

Main Methods:

• Characterization of yeast (Saccharomyces cerevisiae) proteins involved in Sec4 regulation.
• Identification and analysis of suppressor mutations, such as DSS4-1, affecting Sec4 function.
• Genetic interaction studies with sec4 mutations to identify components of the exocytosis pathway.

Main Results:

• A yeast activity comparable to mammalian GDP dissociation inhibitor (GDI) was identified, releasing GDP-bound Sec4 from membranes.
• The DSS4-1 mutation, a dominant suppressor of sec4-8, encodes a nucleotide exchange protein.
• Genetic interactions suggest SEC8 and SEC15 encode proteins forming a complex associated with the plasma membrane, potentially part of the Sec4-regulated exocytosis machinery.

Conclusions:

• Sec4's functional cycle is regulated by nucleotide exchange and dissociation factors, analogous to GDI.
• Dss4 acts as a guanine nucleotide exchange factor, promoting Sec4 cycling.
• The Sec4 cycle is integral to the dynamic assembly and disassembly of protein complexes required for efficient exocytosis.