Yersinia YopE is targeted for type III secretion by N-terminal, not mRNA, signals

Pathogenic Yersinia species inject virulence proteins, known as Yops, into the cytosol of eukaryotic cells. The injection of Yops is mediated via a ty pe III secretion system. Previous studies have suggested that YopE is targe ted for secretion by two signals. One is mediated by its cognate chaperone YerA, whereas the other consists of either the 5' end of yopE mRNA or the N -terminus of YopE. In order to characterize the YopE N-terminal/5' mRNA sec retion signal, the first 11 codons of yopE were systematically mutagenized. Frameshift mutations, which completely alter the amino acid sequence of re sidues 2-11 but leave the mRNA sequence essentially intact, drastically red uce the secretion of YopE in a yerA mutant. In contrast, a mutation that al ters the yopE mRNA sequence, while leaving the amino acid sequence of YopE unchanged, does not impair the secretion of YopE. Therefore, the N-terminus of YopE, and not the 5' end of yopE mRNA, serves as a targeting signal for type III secretion. In addition, the chaperone YerA can target YopE for ty pe III secretion in the absence of a functional N-terminal signal. Mutation al analysis of the YopE N-terminus revealed that a synthetic amphipathic se quence of eight residues is sufficient to serve as a targeting signal. YopE is also secreted rapidly upon a shift to secretion-permissive conditions. This 'rapid secretion' of YopE does not require de novo protein synthesis a nd is dependent upon YerA. Furthermore, this burst of YopE secretion can in duce a cytotoxic response in infected HeLa cells.