Abstract
A multiprotein complex polarisome nucleates actin cables for polarized cell growth in budding yeast and filamentous fungi. However, the dynamic regulations of polarisome proteins in polymerizing actin under physiological and stress conditions remains unknown. We identify a previously functionally unknown polarisome member, actin-interacting-protein 5 (Aip5), which promotes actin assembly synergistically with formin Bni1. Aip5-C terminus is responsible for its activities by interacting with G-actin and Bni1. Through N-terminal intrinsically disordered region, Aip5 forms high-order oligomers and generate cytoplasmic condensates under the stresses conditions. The molecular dynamics and reversibility of Aip5 condensates are regulated by scaffolding protein Spa2 via liquid-liquid phase separation both in vitro and in vivo. In the absence of Spa2, Aip5 condensates hamper cell growth and actin cable structures under stress treatment. The present study reveals the mechanisms of actin assembly for polarity establishment and the adaptation in stress conditions to protect actin assembly by protein phase separation.
Highlights
A multiprotein complex polarisome nucleates actin cables for polarized cell growth in budding yeast and filamentous fungi
Aip[5] co-localized with polarisome members Bni[1] and Bud[6], in which the colocalization was more pronounced in the punctate foci upon Latrunculin A (LatA) treatment (Supplementary Fig. 1c)
A polarisome is a multiprotein complex that is required for polarized yeast growth and the hyphal formation of filamentous fungi by regulating actin cable nucleation[38]
Summary
A multiprotein complex polarisome nucleates actin cables for polarized cell growth in budding yeast and filamentous fungi. Studies of the protein–protein interaction or protein multivalence of actin nucleators and NPFs offer essential implications in understanding the in vivo complex assembly of different types of actin networks within the same crowded cytoplasm. The molecular mechanisms by which polarisome members concentrate in the non-membranous compartment to cooperatively regulate actin assembly are poorly understood, especially under different physiological or stress cues. Structural studies by X-ray crystallography and biochemical assays have identified that Aip5-C is in a dimeric form, in which the dimer state and a loop region that directly binds to G-actin play essential roles in regulating actin assembly activity. To maintain the local protein concentration at the elongating tip, the N-terminal intrinsically disordered region (IDR) of Aip[5] confers unique oligomerization properties, which directly interacts with the C-terminus of polarisome scaffolding protein Spa[2]. Physiological stresses of energy depletion and changing of intracellular pH induce reversible Aip[5] aggregates, in a Spa2-dependent manner, indicating an adaptation mechanism to rescue Aip[5] proteins and dissolve their assemblies rapidly during stress recovery, and ensuring a restart of actin assembly
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