INDIVIDUAL CHAPERONES REQUIRED FOR YOP SECRETION BY YERSINIA

Pathogenic yersiniae secrete anti-host proteins called Yops, by a rece ntly discovered Sec-independent pathway. The Yops do not have a classi cal signal peptide at their N terminus and they are not processed duri ng membrane translocation. The secretion domain is nevertheless contai ned in their N-terminal part but these domains do not resemble each ot her in the different Yops. We have previously shown that YopE secretio n requires SycE, a 15-kDa acidic protein acting as a specific cytosoli c 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-ter minal part of YopH and either the bacterial alkaline phosphatase or th e cholera toxin B subunit. Loss of this protein by mutagenesis led to accumulation of YopH in the cytoplasm and to a severe and selective re duction of YopH secretion. This protein thus behaves like the counterp art of SycE and we called it SycH. We also engineered a mutation in lc rH, the gene upstream from yopB and yopD, known to encode a 19-kDa aci dic protein. Although this mutation was nonpolar, the mutant no longer secreted YopB and YopD. The product of lcrH could be immunoprecipitat ed together with cytoplasmic YopD. lcrR therefore seems to encode a Yo pD-specific chaperone, which we called SycD. Determination of the depe ndence of YopB on SycD requires further investigation. SycE, SycH, and SycD appear to be members of a new family of cytosolic chaperones req uired for Yop secretion.