Chlamydia pneumoniae is an important human and animal pathogen. Infection by this obligate intracellular bacterial pathogen critically depends on host-cell structures for its entry, establishment and growth. All Chlamydia species start infection with the adhesion of the non-metabolic extracellular infectious form (the elementary body, EB) to the host cell. Adhesion is followed by internalization of the pathogen but the molecular mechanisms of this step of the infection are only partly understood. Attachment of C. pneumoniae to host cells requires binding of the bacterial adhesins OmcB and Pmp21 to host cell glycosaminoglycans and EGF growth factor receptor, respectively. Intriguingly, at the EB/host cell contact site phosphatidylserine (PS) is externalized from the inner to the outer leaflet of the host cell membrane indicative of massive changes in membrane identity accompanying chlamydial host cell entry. A newly identified EB cell surface protein, which we named LIPP, adheres to human cells, enhances chlamydial infection by stimulating EB internalization, and most importantly directly binds PS and phosphatidic acid (PA) in artificial membranes. Recombinant LIPP is sufficient for PS externalization at the host cell membrane. Thus, chlamydial LIPP modulates the host cell plasma membrane by inducing PS externalization to promote the infection. How this is achieved mechanistically is the subject of this proposal. We will uncover the LIPP-mediated molecular mechanisms of modulating the identity, dynamics and composition of the plasma membrane and the associated protein complexes at the side of an invading Chlamydia.