Scube2 enhances proteolytic Shh processing from the surface of Shh-producing cells
All morphogens of the Hedgehog (Hh) family are synthesized as dually lipidated proteins, which results in their firm attachment to the surface of producing cells. Thus, Hh release into the extracellular space requires accessory protein activities. We previously suggested proteolytic removal of N- and C-terminal lipidated peptides (shedding) to be one such activity. More recently, the secreted glycoprotein Scube2 (signal peptide, cubulin domain, epidermal growth factor-like protein 2) was also implicated in Shh release from the cell membrane. This activity strictly depended on Scube2 CUB domains, which derive their name from the complement serine proteinases and bone morphogenetic protein-1/tolloid metalloproteinases C1r/C1s, Uegf and Bmp1 and function as proteolytic activity regulators in these proteins. This suggested that sheddases and Scube2 may co-operate in Shh release. We here confirm that sheddases and Scube2 act co-operatively to increase the pool of soluble bioactive Shh and that Scube2-dependent morphogen release is unequivocally linked to the proteolytic processing of lipidated Shh termini, resulting in truncated soluble Shh. Scube2 proteins thus act as protease enhancers in this setting, revealing novel Scube2 functions in Hh signalling regulation.