| Category | Summary |
|---|---|
| Identity/complex | **SPA2 / YLL021W / UniProt P23201** in *Saccharomyces cerevisiae* encodes **Spa2**, a **polarisome scaffold protein** and cell-polarity factor; Spa2 is a core member of the **polarisome** with Bni1, Bud6, and Pea2, organizing polarized growth and mating projection formation rather than acting as an enzyme. (pqac-00000001, pqac-00000003, pqac-00000007) |
| Domains/regions | Spa2 is a **multi-domain scaffold** with annotated **Spa2 homology domains (SHD-I to SHD-V)**, including an N-terminal SHD important for protein interactions and a region/function corresponding to **SHD-II** needed for targeting to polarized growth sites; recent work also defines an **N-terminal intrinsically disordered region (aa 1–281)** that mediates condensation/phase separation and a **dimeric nucleation core (aa 281–535)** that nucleates ADP-actin. (pqac-00000004, pqac-00000006, pqac-00000008) |
| Key molecular functions | Primary function is **scaffolding/organization of polarized growth machinery**: Spa2 helps focus the polarisome at cortical growth zones, promotes **formin-dependent actin cable assembly**, recruits or stabilizes partners, and under glucose starvation can **directly remodel ADP-actin** through nucleation plus condensation-mediated bundling. (pqac-00000001, pqac-00000006, pqac-00000008) |
| Key interactors | Experimentally supported partners include **Bni1** and **Bud6** within the polarisome; **Aip5**, whose polarized localization depends on Spa2 C-terminus; **Msb3/4** and **Sec4** in a positive-feedback pathway affecting Bud6 delivery; and cytokinetic factors **Cyk3, Hof1, Chs2**, plus associations with **Myo1/Myo2**, IPC components, and septin-linked machinery at division sites. (pqac-00000001, pqac-00000004, pqac-00000005, pqac-00000007) |
| Subcellular localization | Spa2 localizes dynamically to **incipient bud sites, bud tips, mating projection (shmoo) tips, bud neck/division sites**, and under energy/glucose starvation redistributes from the bud tip into **filamentous structures/puncta overlapping actin cables**. (pqac-00000003, pqac-00000005, pqac-00000006, pqac-00000010) |
| Biological processes | Spa2 functions in **cell polarity establishment and maintenance**, **polarized secretion/exocytosis**, **actin cable organization**, **bud growth**, **pheromone-induced morphogenesis/mating**, and additionally **cytokinesis/primary septum formation** by coordinating Chs2 incorporation at the cleavage site. (pqac-00000001, pqac-00000002, pqac-00000005) |
| Recent (2023-2024) findings | Recent literature emphasizes **biomolecular condensation/phase separation** in fungal polarity systems, including Spa2-centered regulation of polarisome behavior; a major 2024 study showed that S. cerevisiae Spa2 **specifically remodels ADP-actin during glucose starvation** via an IDR-dependent condensation mechanism and nucleation core, expanding Spa2 from a scaffold-only model to an active stress-responsive actin remodeler. Spa2 also remains relevant in 2024 cytokinesis reviews as a factor recruited early to the division site to coordinate septum machinery. (pqac-00000008, pqac-00000010) |
| Quantitative data points | In 2024 experiments, **cellular ATP dropped by ~38% within 5 min** of glucose starvation, coinciding with rapid **actin cable bundling at 5 min** and recovery by ~30 min; **Spa2(1–535)** was identified as the **minimal truncation** retaining starvation-triggered filament formation and bundle stabilization, whereas isolated **1–281** or **281–535** fragments were insufficient for full activity. A 2023 evolutionary-engineering study also reported a **Spa2 A658T** mutation in a strain with **~3-fold improved tolerance to 2-phenylethanol (3.4 g/L)**, although causality for SPA2 specifically was not established. (pqac-00000008, pqac-00000010) |


*Table: This table summarizes the verified identity, molecular role, localization, pathways, and recent findings for *S. cerevisiae* Spa2 (UniProt P23201). It is useful as a compact evidence-backed functional annotation for the yeast polarisome scaffold.*