Key regulator of transforming growth factor beta (TGFB1, TGFB2 and TGFB3) that controls TGF-beta activation by maintaining it in a latent state during storage in extracellular space (PubMed: 2022183, PubMed: 8617200, PubMed: 8939931). Associates specifically via disulfide bonds with the Latency-associated Peptide (LAP), which is the regulatory chain of TGF-beta, and regulates integrin-dependent activation of TGF-beta (PubMed: 8617200, PubMed: 8939931, PubMed: 15184403). Outcompeted by LRRC32/GARP for binding to LAP regulatory chain of TGF-beta (PubMed: 22278742).