Analysis of the haemolysin secretion system by PhoA-HlyA fusion proteins

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Abstract
We studied the efficiency of the pHly152-derived haemolysin transport system using PhoA-HlyA fusion proteins and different constructs which provide HlyB/HlyD in trans. The optimal C-terminal HlyA signal consists of the last 60 amino acids. Longer stretches of HlyA do not improve the transport efficiency of PhoA-HlyA fusion proteins. The introduction of deletions and/or replacements in the 60 amino acid HlyA signal domain revealed at least three functional regions with different degrees of specificity. Amino acids 1–21 (numbered from the N-terminal part of the 60 amino acid HlyA signal), termed region I, could be replaced by a Pro-containing peptide. The other two regions II and III (amino acids 22–40 and 41–60, respectively) seem to interact directly with the HlyB/HlyD translocator since a PhoA fusion protein which contains either of the two regions was still secreted in a HlyB/HlyD-dependent mode, albeit at low efficiency. An efficient trans-complementing HlyB/HlyD system was only obtained from the pHLy152-encoded hly determinant when the regulatory hlyR element was provided in cis. Secretion of the PhoA-HlyA fusion protein did not interfere with the secretion of HlyA even when the fusion protein was induced to a high level. This suggests that the capacity of the HlyB/HlyD translocation system is high and not normally saturated by its natural HlyA substrate.