YEAST AIM21/TDA2 REDUCES BARBED END ASSEMBLY TO MAINTAIN FREE ACTIN POOL AND BALANCE ACTIN BETWEEN PATCHES AND CABLES

Abstract

How cells balance the incorporation of actin into diverse structures is poorly understood. In budding yeast, a single actin monomer pool is used to build both actin cables involved in polarized growth and actin cortical patches involved in endocytosis. Here I report how Aim21/Tda2 is recruited to the membrane-proximal region of actin patches, how it negatively regulates actin assembly at patches to elevate the available actin monomer pool, and how an array of actin regulators is orchestrated to ensure proper assembly of both structures. Aim21 has four polyproline regions and is recruited to actin patches by two SH3-containing patch proteins Bbc1 and Abp1. The C-terminal region, which is required for its function, binds to Tda2. Cell biological and biochemical data reveal that Aim21/Tda2 is a negative regulator of barbed end F-actin assembly and plays a pivotal role in balancing the distribution of actin between cables and patches, demonstrated by the rescue of the growth defect of tpm1Δ cells by aim21Δ. Additionally, this activity is necessary for efficient endocytosis. Aim21/Tda2 forms a complex with the F-actin barbed end capping protein Cap1/Cap2, revealing an interplay between regulators, and illustrating the complexity of regulation of barbed end assembly.