Malfolded recombinant Tat substrates are Tat-independently degraded in Escherichia coli

The twin-arginine translocation (Tat) system translocates folded proteins across biological membranes. It has been suggested that the Tat system of Escherichia coli can direct Tat substrates to degradation if they are not properly folded [Matos, C.F., Robinson, C. and Di Cola, A. (2008) The Tat system proofreads FeS protein substrates and directly initiates the disposal of rejected molecules. EMBO J. 27, 2055–2063; Matos, C.F., Di Cola, A. and Robinson, C. (2009) TatD is a central component of a Tat translocon-initiated quality control system for exported FeS proteins in Escherichia coli. EMBO Rep. 10, 474–479]. Contrary to the earlier reports, it is now concluded that reported differences between tested strains were due to variations in expression levels and inclusion body formation. Using the native Tat substrate NrfC and a malfolded variant thereof, we show that the turnover of these proteins is not affected by the absence of all known Tat components. Malfolded NrfC is degraded more quickly than the native protein, indicating that Tat-independent protease systems can recognize malfolded Tat substrates.