Transforming growth factor beta (TGF-beta) is a family of three cytokine ‘isoforms’ (encoded by three separate human genes) that control proliferation, cellular differentiation and other functions. TGF-beta originally referred to the founding member TGF-beta-1, now it is often used as a collective term for all three. TGF-beta is secreted from cells in latent form as part of a complex that includes two other proteins: the cleaved propeptide of TGF beta, known as latency associated peptide (LAP), and a member of the latent TGF beta binding protein (LTBP) family. LTBPs are members of the fibrillin/LTBP superfamily, characterised by the presence of unique TGF-binding protein (TB) domains, also known as 8 cys domains as they contain eight characteristic cysteines (Ramirez & Sakai 2010). LTBPs are microfibril-associated proteins that tether latent complexes of TGF-beta to microfibrils in the ECM (Taipale et al. 1996, Dallas et al. 2000, Isogai et al. 2003, Hyytiainen et al. 2004, Ono et al. 2009, Munger & Sheppard 2011). This allows TGF-beta to be targeted to the ECM where it is maintained in an inactive, latent state (Robertson et al. 2011).
LTBP1 and 3 bind all three isoforms of latent TGF-beta, while LTBP4 only weakly binds TGF-beta1 (Saharinen & Keski Oja 2000). LTBP2 does not bind TGF-beta and is a structural component of fibrillin microfibrils. The carboxyl termini of LTBP1 and LTBP4 binds to fibrillin. The incorporation of LTBP1 and LTBP4 into the ECM is abolished in fibrillin-1 null mice (Ono et al. 2009). The amino terminus of LTBPs binds ECM components such as collagen (Taipale et al. 1996) and fibronectin (Kantola et al. 2008). Fibulins compete for the LTBP sites in fibrillin (Ono et al. 2009).