N-terminal cleavage of proTGFα occurs at the cell surface by a TACE-independent activity

ProTGFα (transforming growth factor α precursor) maturation and conversion into soluble TGFα is a complex process that involves three proteolytic steps. One, that occurs co-translationally, eliminates the signal sequence. Another, occurring at the juxtamembrane domain, solubilizes TGFα. A third cleavage removes the N-terminal extension of proTGFα. This latter step has been poorly studied, mainly because of the rapid kinetics of this cleavage. In the present study, we have designed a strategy to analyse several aspects regarding this N-terminal cleavage. In vivo treatment with the hydroxamate-based metalloprotease inhibitors BB3103 or TAPI-2 (tumour necrosis factor-α protease inhibitor 2) reversibly induced accumulation of forms of proTGFα that included the N-terminal extension. N-terminal shedding was rapid, and occurred at the cell surface. However, the machinery responsible for the N-terminal cleavage was inactive in other cellular sites, such as the endoplasmic reticulum. Experiments of proTGFα expression and maturation in cells deficient in TACE (tumour-necrosis-factor-α-converting enzyme) activity indicated that this protease was dispensable for N-terminal processing of proTGFα in vivo, but was required for regulated cleavage at the C-terminus. These findings indicate that TACE is not involved in N-terminal processing of proTGFα, and suggest differences in the machineries that control the cleavage at both ends of TGFα within its precursor.