FAM89B functions as a signalling adapter with two described roles. It negatively regulates TGF-beta-induced signalling: in cooperation with SKI, it prevents the translocation of SMAD2 from the nucleus to the cytoplasm in response to TGF-beta. NCBI Gene also notes predicted transcription corepressor binding activity and a predicted role in negative regulation of the transmembrane receptor protein serine/threonine kinase signalling pathway.

In the lamellipodium, FAM89B (as LRAP25) mediates the specific recognition of LIMK1 by CDC42BPA and CDC42BPB. This targeting of CDC42BPA/CDC42BPB to LIMK1 and to the lamellipodium results in LIMK1 activation and subsequent phosphorylation of CFL1, which is important for the regulation of lamellipodial F-actin. Predicted involvement in the establishment of cell polarity and positive regulation of cell migration is consistent with this activity at the cell edge.

The FAM89B gene belongs to the FAM89 family and is annotated as active in the cytoplasm and lamellipodium. The protein is listed in UniProt as leucine repeat adapter protein 25, abbreviated LRAP25, and the gene has also carried the alias MTVR1. Its HGNC name, family with sequence similarity 89 member B, reflects its original classification by sequence similarity. Mouse and rat orthologs are both designated Fam89b.