The cytosolic form of aspartate aminotransferase is required for full activation of TOR complex 1 in fission yeast

Abstract

The evolutionarily conserved TOR complex 1 (TORC1) activates cell growth and proliferation in response to nutritional signals. In the fission yeast Schizosaccharomyces pombe, TORC1 is essential for vegetative growth, and its activity is regulated in response to nitrogen quantity and quality. Yet, how TORC1 senses nitrogen is poorly understood. Rapamycin, a specific TOR inhibitor, inhibits growth in S. pombe only under conditions in which the activity of TORC1 is compromised. In a genetic screen for rapamycin-sensitive mutations, we isolated caa1-1, a loss-of-function mutation of the cytosolic form of aspartate aminotransferase (Caa1). We demonstrate that loss of caa1⁺ partially mimics loss of TORC1 activity and that Caa1 is required for full TORC1 activity. Disruption of caa1⁺ resulted in aspartate auxotrophy, a finding that prompted us to assess the role of aspartate in TORC1 activation. We found that the amino acids glutamine, asparagine, arginine, aspartate, and serine activate TORC1 most efficiently following nitrogen starvation. The glutamine synthetase inhibitor l-methionine sulfoximine abolished the ability of asparagine, arginine, aspartate, or serine, but not that of glutamine, to induce TORC1 activity, consistent with a central role for glutamine in activating TORC1. Neither addition of aspartate nor addition of glutamine restored TORC1 activity in caa1-deleted cells or in cells carrying a Caa1 variant with a catalytic site substitution, suggesting that the catalytic activity of Caa1 is required for TORC1 activation. Taken together, our results reveal the contribution of the key metabolic enzyme Caa1 to TORC1 activity in S. pombe.

Keywords: Caa1; Psk1; S6 kinase; Schizosaccharomyces pombe; TOR complex (TORC); aspartate amino acid transferase; glutamine synthase; nitrogen sensing; nutrient signaling; serine/threonine protein kinase; target of rapamycin (TOR).