Functions as a negative regulator of TGF-beta signaling and thereby probably plays a role in cell proliferation, differentiation, apoptosis, motility, extracellular matrix production and immunosuppression. In the canonical TGF-beta pathway, ZFYVE9/SARA recruits the intracellular signal transducer and transcriptional modulators SMAD2 and SMAD3 to the TGF-beta receptor. Phosphorylated by the receptor, SMAD2 and SMAD3 then form a heteromeric complex with SMAD4 that translocates to the nucleus to regulate transcription. Through interaction with SMAD2 and SMAD3, LDLRAD4 may compete with ZFYVE9 and SMAD4 and prevent propagation of the intracellular signal. Interacts with PMEPA1. Interacts (via the SMAD interaction motif) with SMAD2 and SMAD3. Expressed in lymphocytes. The SMAD interaction motif is required for interaction with SMAD2 and SMAD3 and the negative regulation of TGF-beta signaling. Belongs to the PMEPA1 family.