Pathogenic yersiniae secrete anti-host proteins called Yops, by a recently discovered Sec-independent pathway. The Yops do not have a classical signal peptide at their N terminus and they are not processed during membrane translocation. The secretion domain is nevertheless contained in their N-terminal part but these domains do not resemble each other in the different Yops. We have previously shown that YopE secretion requires SycE, a 15-kDa acidic protein acting as a specific cytosolic chaperone. Here we show that the gene downstream from yopH encodes a 16-kDa acidic protein that binds to hybrid proteins made of the N-terminal part of YopH and either the bacterial alkaline phosphatase or the cholera toxin B subunit. Loss of this protein by mutagenesis led to accumulation of YopH in the cytoplasm and to a severe and selective reduction of YopH secretion. This protein thus behaves like the counterpart of SycE and we called it SycH. We also engineered a mutation in lcrH, the gene upstream from yopB and yopD, known to encode a 19-kDa acidic protein. Although this mutation was nonpolar, the mutant no longer secreted YopB and YopD. The product of lcrH could be immunoprecipitated together with cytoplasmic YopD. lcrH therefore seems to encode a YopD-specific chaperone, which we called SycD. Determination of the dependence of YopB on SycD requires further investigation. SycE, SycH, and SycD appear to be members of a new family of cytosolic chaperones required for Yop secretion.
