It was previously shown that expression of an activated Phactr1 mutant (Phactr1XXX) that constitutively forms the Phactr1/PP1 phosphatase holoenzyme, induces F-actin rearrangements in NIH3T3 fibroblasts. Expression of the Phactr1 PP1-binding domain (C-terminal) alone is also sufficient to induce such cytoskeletal changes. In contrast, expression of Phactr1XXXC derivative, which lacks the PP1 binding sequences does not result in alteration of cytoskeletal morphology (Wiezlak et al., 2012). These observations suggest that Phactr1/PP1 dephosphorylates target proteins involved in cytoskeletal dynamics. To identify potential Phactr1/PP1 substrates, we used differential SILAC phosphoproteomics in NIH3T3 cells inducibly expressing Phactr1XXX, Phactr1XXXC constructs or vector alone. Over 3000 phosphorylation sites were quantified, among which we determined Phactr1/PP1 target dephosphorylation sites by comparative analysis.