pilot protein

[bioinf]

Hi there,
I am currently looking into the type III secretion system, and am trying to unravel what the protein YscW does. I can find literature on it, and it says that it is a pilot protein: "YscW expression increased the yield of oligomeric YscC and was required for its outer membrane localization, confirming the function of YscW as a pilot protein." However i'm struggling to get a clear picture in my head what a pilot protein is. Can anyone enlighten me?

Thanks.