Actin filaments and microtubules are cytoskeletal polymers that participate in many vital cell functions including division, morphogenesis, phagocytosis, and motility. Despite the persistent dogma that actin filament and microtubule networks are distinct in localization, structure, and function, a growing body of evidence shows that these elements are choreographed through intricate mechanisms sensitive to either polymer. Many proteins and cellular signals that mediate actin–microtubule interactions have already been identified. However, the impact of these regulators is typically assessed with actin filament or microtubule polymers alone, independent of the other system. Further, unconventional modes and regulators coordinating actin–microtubule interactions are still being discovered. Here we examine several methods of actin–microtubule crosstalk with an emphasis on the molecular links between both polymer systems and their higher-order interactions.

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Profilin-1 (PFN1) is a cytoskeletal protein that regulates the dynamics of actin and microtubule assembly. Thus, PFN1 is essential for the normal division, motility, and morphology of cells. Unfortunately, conventional fusion and direct labeling strategies compromise different facets of PFN1 function. As a consequence, the only methods used to determine known PFN1 functions have been indirect and often deduced in cell-free biochemical assays. We engineered and characterized two genetically encoded versions of tagged PFN1 that behave identical to each other and the tag-free protein. In biochemical assays purified proteins bind to PIP lipids, catalyze nucleotide exchange on actin monomers, stimulate formin-mediated actin filament assembly, and impact microtubule dynamics. Halo-PFN1 or mApple-PFN1 restored morphological and cytoskeletal functions in PFN1-deficient mammalian cells. In biochemical assays, mAp-PFN1 bound tubulin dimers (kD = 1.89 µM) and the sides of microtubules in vitro. Titrations of self-labeling Halo-ligands were used to visualize molecules of PFN1. This approach combined with specific function-disrupting point-mutants (Y6D and R88E) revealed PFN1 bound to microtubules in live cells. Cells expressing the ALS-associated G118V disease variant did not associate with actin filaments or microtubules. Thus, these tagged PFN1s are reliable tools for studying the dynamic interactions of PFN1 with actin or microtubules in vitro as well as in important cell processes or disease-states.

Profilin is an essential regulator of actin and microtubule dynamics and therefore a critical control point for the normal division, motility, and morphology of cells. Most studies of profilin have focused on biochemical investigations using purified protein because high cellular concentrations (121 uM) present challenges for conventional imaging modalities. In addition, past studies that employed direct labeling or conventional fusion protein strategies compromised different facets of profilin function. We engineered a fluorescently-labeled profilin that retains native activities with respect to phosphoinositide lipids, actin monomers, formin-mediated actin assembly, and microtubule polymerization. This fluorescent profilin directly binds to dimers of tubulin (kD = 1.7 uM) and the microtubule lattice (kD = 10 uM) to stimulate microtubule assembly. In cells, our tagged profilin fully rescues profilin-1(-/-) cells from knockout-induced perturbations to cell shape, actin filament architecture, and microtubule arrays. Thus, this labeled profilin-1 is a reliable tool to investigate the dynamic interactions of profilin with actin or microtubules in live cell and in vitro applications.