Legionella pneumophila is a water-borne bacterial pathogen which can cause severe and often fatal pneumonia. In order to survive within phagocytic host cells, Legionella secretes hundreds of different effector proteins through its Type IV secretion system (T4SS). Soluble and weakly hydrophobic transmembrane domain (TMD)-containing effector proteins might be secreted directly using a one-step secretion pathway, while TMD-containing effectors with a higher hydrophobicity may be targeted by the signal recognition particle (SRP) and integrated into the bacterial inner membrane before being accepted by the T4SS in a two-step secretion pathway. For a better understanding of how the SRP recognizes the strong hydrophobic TMD-containing effectors I currently establish a new method to be able to identify potential SRP binding sites.