The Architecture of the Rag GTPase Signaling Network

Abstract

The evolutionarily conserved target of rapamycin complex 1 (TORC1) couples an array of intra- and extracellular stimuli to cell growth, proliferation and metabolism, and its deregulation is associated with various human pathologies such as immunodeficiency, epilepsy, and cancer. Among the diverse stimuli impinging on TORC1, amino acids represent essential input signals, but how they control TORC1 has long remained a mystery. The recent discovery of the Rag GTPases, which assemble as heterodimeric complexes on vacuolar/lysosomal membranes, as central elements of an amino acid signaling network upstream of TORC1 in yeast, flies, and mammalian cells represented a breakthrough in this field. Here, we review the architecture of the Rag GTPase signaling network with a special focus on structural aspects of the Rag GTPases and their regulators in yeast and highlight both the evolutionary conservation and divergence of the mechanisms that control Rag GTPases.

Keywords: EGO complex; Lst4–Lst7; Rag GTPases; SEACAT; SEACIT; amino acid signaling; budding yeast; target of rapamycin complex 1 (TORC1).

Figure 1

(a) Alignment of the G domain sequences of budding yeast (Gtr1 and Gtr2) and human (RagA, RagB, RagC, and RagD) Rag GTPases together with the G domain of human Ras (H-Ras). Conserved residues are highlighted in grey. Boxes denote typical GTPase sequence motifs (see text). Arrow heads mark important residues discussed in the text. Red arrow heads highlight typical Rag GTPase residues; (b) Schematic representation of the sequences of yeast and human Rag GTPases together with H-Ras. The sequences were aligned with respect to their G domains.

Figure 2

(a) Cartoon representation of the structure of the heterodimeric Gtr1–Gtr2 complex ([26], PDB entry 3R7W); (b) Details of the G domains of Gtr1 and Gtr2. The red regions correspond to the P-loop and the SW I/SW II domains in each protein. Important residues discussed in the text or listed in Table 1 are labeled. The Mg²⁺ atom is shown as a black sphere.

Figure 3

Schematic representation of the vacuolar membrane-bound budding yeast Ego1–Ego2–Ego3 ternary complex (EGO-TC), and of the conformational change of the EGO-TC-associated Gtr1–Gtr2 heterodimer following the exchange of GDP for GTP on Gtr1 and GTP hydrolysis by Gtr2 (upper arrow) and vice versa (lower arrow).

Figure 4

The Rag GTPase signaling network in yeast and mammals. (a,b) Upstream regulators that antagonize (when amino acids are limiting; upper panels) or stimulate (when amino acids are abundant; lower panels) the Rag GTPase- target of rapamycin complex 1 (TORC1) signaling branch in yeast (a) and mammalian (b) cells. Red and blue arrows indicate GTP exchange factors (GEF) and GTPase activating protein (GAP) activities, respectively. Arrows and bars denote activating and inhibiting activities, respectively. Dashed arrows and question marks indicate mechanisms that are currently only partially understood. For further details, see text.