Characterization of protein-protein interactions (PPIs) is a fundamental goal in the post-genomic era. Here, we document a generally applicable approach to identify cellular protein interactomes using a combination of nanobody-based affinity purification (AP) coupled with liquid chromatography and tandem mass spectrometry (LC-MS/MS). The Hippo signaling regulator TAZ (also known as WWTR1) functions as a transcriptional co-repressor or activator depending on its PPI network; we therefore undertook an unbiased proteomic screen to identify TAZ PPIs in striated muscle cells. A GFP nanotrap-based AP approach coupled with protein identification through LC-MS/MS was used to document a comprehensive list of known and novel TAZ interactome components. Informatic analysis of the interactome documented known components of the Hippo signaling pathway and multiple epigenetic regulators such as the NuRD, FACT and SWI/SNF complexes and the pro-myogenic CARM1 methyltransferase. Hippo pathway reporter gene (HOP/HIP) analysis indicated that CARM1 represses TAZ transcriptional co-activator function, promoting TAZ Ser89 phosphorylation and TAZ cytoplasmic sequestration. MS analysis revealed that CARM1 dimethylates TAZ at Arg77 in a PGPR*LAGG consensus peptide, resulting in enhanced TAZ Ser89 phosphorylation. These studies underline the utility of a nanobody-based AP approach for interactome analysis.