Structure and function of the yeast amino acid-sensing SEAC-EGOC supercomplex.

Area of Science:

• Cellular signaling
• Molecular mechanisms of cell growth regulation
• Amino acid sensing pathways

Background:

• The Seh1-associated complex (SEAC), known as GATOR in mammals, is critical for transducing amino acid availability signals to the Target of Rapamycin Complex 1 (TORC1).
• SEAC comprises SEACIT (GATOR1), a TORC1 inhibitor with GTPase-activating protein (GAP) activity towards Gtr1, and SEACAT (GATOR2), which modulates SEACIT activity.
• The precise molecular mechanisms by which SEACAT regulates SEACIT remain incompletely understood.

Purpose of the Study:

• To elucidate the structural basis of SEAC-EGOC interaction using cryo-electron microscopy.
• To investigate the functional role of the SEAC-EGOC complex in amino acid signaling to TORC1.
• To identify key components within SEACAT, particularly Sea2, involved in regulating TORC1 pathway activation.

Main Methods:

• Cryo-electron microscopy (cryo-EM) to determine the structure of the SEAC-EGOC complex.
• Biochemical assays to assess the GTPase-activating protein (GAP) activity of SEACIT.
• Genetic studies involving subunit deletions (e.g., Sea2) to evaluate effects on amino acid signaling to TORC1.

Main Results:

• The cryo-EM structure reveals that SEACIT directly binds to two EGOC molecules in their active conformation, independent of SEACAT.
• The GAP activity of SEACIT is essential for amino acid-dependent TORC1 regulation, and its disruption mimics the phenotype of Gtr1-Gtr2 deficiency.
• Loss of Sea2, or its N-terminal β-propeller domain, significantly impairs amino acid signaling to TORC1, suggesting its role in recruiting a GAP inhibitor.

Conclusions:

• The SEAC-EGOC complex functions as a central hub for sensing amino acids and relaying signals to TORC1.
• The Sea2 β-propeller domain likely mediates rapid amino acid signaling to TORC1 by recruiting a GAP inhibitor, with additional slower regulatory pathways also implicated.