Processing of Transforming Growth Factor β1 Precursor by Human Furin Convertase

Proteolytic processing of the transforming growth factor β precursor (pro-TGFβ) is an essential step in the formation of the biologically active TGFβ homodimeric protein (TGFβ). The 361-amino-acid precursor pro-TGFβ1 has within its primary structure the R-H-R-R processing signal found in many constitutively secreted precursor proteins and potentially recognized by members of the mammalian convertase family of endoproteases. To determine whether cleavage of pro-TGFβ1 can be achieved by the furin convertase in vitro, purified precursor was incubated in the presence of a truncated/secreted form of the enzyme. Immunoblots showed that the 55-kDa pro-TGFβ1 was converted into the 44 and 12.5 kDa bands corresponding to the pro-region and the mature monomer, respectively. Treatment of pro-TGFβ1 with furin resulted in a 5-fold increase in the production of biologically active TGFβ1. Furthermore, when expressed in the furin-deficient LoVo cells, no processing of pro-TGFβ1 was observed. In contrast, efficient processing was oberved when pro-TGFβ was coexpressed with the furin convertase. Collectively, these results provide evidence that in our experimental systems the TGFβ1 precursor is efficiently and correctly processed by human furin thus permitting release of the biologically active peptide.