It belongs to the SMAD family of proteins, which belong to the  TGFβ superfamily of modulators. Like many other TGFβ family members SMAD5 is involved in cell signalling and modulates signals of bone morphogenetic proteins (BMP's).
The protein in question belongs to the SMAD family, a group within the TGFβ superfamily that acts as a modulator. Similar to other TGFβ family members, SMAD5 plays a role in cellular signaling and influences BMP-related signals.