Hydrophilic microenvironment required for the channel-independent insertase function of YidC protein

The recently solved crystal structure of YidC protein suggests that it mediates membrane protein insertion by means of an intramembrane cavity rather than a transmembrane (TM) pore. This concept of protein translocation prompted us to characterize the native, membrane-integrated state of YidC with respect to the hydropathic nature of its TM region. Here, we show that the cavity-forming region of the stage III sporulation protein J (SpoIIIJ), a YidC homolog, is indeed open to the aqueous milieu of the Bacillus subtilis cells and that the overall hydrophilicity of the cavity, along with the presence of an Arg residue on several alternative sites of the cavity surface, is functionally important. We propose that YidC functions as a proteinaceous amphiphile that interacts with newly synthesized membrane proteins and reduces energetic costs of their membrane traversal.