Alpha subunit of the heterotrimeric guanine nucleotide-binding protein (G protein) that mediates mating pheromone signal transduction. Binding of alpha-factor or a-factor to its cognate transmembrane receptor STE2 and STE3, respectively, allows the receptor to serve as a guanine nucleotide exchange factor (GEF) on GPA1. The exchange of GDP for GTP on the G protein alpha subunit alters its interaction with the G protein beta subunit STE4, leading to dissociation of the G protein beta-gamma dimer STE4-STE18. The dissociated subunits activate downstream effectors to activate the mating response pathway and induce changes necessary to produce mating-competent cells. STE4-STE18 activate the downstream pheromone signaling MAP kinase cascade leading to expression of mating-specific genes, inducing cell cycle arrest in G1, promoting polarized cell growth to form mating projections (shmoos), and establishing the changes in plasma membrane, cell wall and nuclear envelope to permit cell-cell fusion (plasmogamy) and fusion of the two haploid nuclei (karyogamy). GPA1 transmits a signal that requires direct binding to the effector enzyme PI3K located at the endosome, promoting increased PI3 production. The intrinsic GTPase activity of GPA1 determines the duration of signaling, and is dramatically accelerated by the RGS protein SST2. In unstimulated cells, GDP-bound GPA1 sequesters the G protein beta-gamma subunit STE4-STE18, preventing it from activating the downstream effectors. Also down-regulates the signal by inhibiting the pheromone-induced accumulation of FUS3 in the nucleus. Alternates between an inactive form bound to GDP and an active form bound to GTP. Activated by the G protein coupled receptors (GPCRs) STE2 and STE3, which serve as guanine nucleotide-exchange factors (GEFs), and inactivated by SST2, probably acting as a GTPase-activating protein (GAP). G proteins are composed of 3 units; alpha, beta and gamma. The alpha chain contains the guanine nucleotide binding site. In its GDP-bound form, binds to the G protein beta-gamma dimer STE4-STE18. Directly interacts with the beta subunit STE4. Probably forms preactivation complexes with unligated receptors STE2 and STE3. Interacts with FUS3. Pheromone-induced activation of GPA1 increases its association with FUS3. Interacts with SCP160. SCP160 binds specifically to the GTP-bound form of GPA1. Interacts with the phoshpatidylinositol 3-kinase (PI3K) subunits VPS15 and VPS34 at the endosome. The GTP-bound form of GPA1 binds directly and selectively to the catalytic subunit VPS34, while the GDP-bound form binds to VPS15, which appears to function as an alternative G protein beta subunit for GPA1. Interacts with regulators of G protein signaling (RGS) proteins MDM1, RAX1, RGS2 and SST2, but SST2 alone binds preferentially to the transition state conformation of GPA1, indicating that it acts as a GAP for this G protein. Localizes predominantly to the plasma membrane in its inactive, GDP-bound form, and is directed to endosomes once in its active, GTP-bound form. Concentrates at the tip of the mating projections. Contains an 'insertion' sequence of 109 residues which is not present in other G-protein alpha chains. N-myristoylation by NMT1 is pheromone-stimulated and required for palmitoylation of Cys-3. This lipid modification anchors the protein to membranes. Depalmitoylated by YLR118C/APT1. Monoubiquitination targets the protein for degradation to the vacuole, and polyubiquitination tags the protein for degradation by the proteasome. This may be an additional signaling regulation mechanism. Present with 9920 molecules/cell in log phase SD medium. Belongs to the G-alpha family. G(q) subfamily.