Autotransporters (ATs) of Gram-negative bacteria are often produced with an unusual signal peptide that carries a conserved N-terminal extension. Using combined in vitro and in vivo approaches we show that the extension of the AT hemoglobin protease (Hbp) does not affect targeting of Hbp via the SRP-pathway, suggesting that the extension is not involved in targeting pathway selection. (c) 2008 Elsevier Inc. All rights reserved.