Obligate human-pathogenic Neisseria gonorrhoeae expresses numerous variant surface proteins mediating adherence to and invasion of target cells. The invariant major outer membrane porin PorB of serotype A (P.IA) gonococci triggers invasion into Chang cells only if the medium is devoid of phosphate. Since gonococci expressing PorB IA are frequently isolated from patients with severe disseminating infections, the interaction initiated by the porin may be of major relevance for the development of this serious disease. Here, we investigated the low-phosphate-dependent invasion and compared it to the well-known pathways of entry initiated by Opa proteins. P.IA-triggered invasion requires clathrin-coated pit formation and the action of actin and Rho GTPases. However, in contrast to Opa-initiated invasion via heparan sulfate proteoglycans, microtubules, acidic sphingomyelinase, phosphatidylinositol 3-kinase, and myosin light chain kinase are not involved in this entry pathway. Nor are Src kinases required, as they are in invasion, e.g., via the CEACAM3 receptor. Invasion by PorB IA occurs in a wide spectrum of cell types, such as primary human epithelial and endothelial cells and in cancer cells of human and animal origin. Low-phosphate-dependent invasion is thus a pathway of gonococcal entry distinct from Opa-mediated invasion.Due to emerging antibiotic resistances, Neisseria gonorrhoeae, the etiological agent of the sexually transmitted disease gonorrhea, is still a serious threat to world health, with an estimated 62 million infections occurring each year. During the infection process, the human-specific pathogen colonizes mucosal tissues and gains access to deeper tissues. In most cases the infection remains localized. However, in approximately 1 to 3% of cases, the gonococcus spreads within the host organism and causes systemic infections leading to serious conditions, such as endocarditis, meningitis, and pneumonia. The gonococcus expresses a remarkable variety of adhesins, like the type IV pili (28, 36), different phase-variable colony opacityassociated (Opa) outer membrane proteins (35) and the principal outer membrane protein, PorB (P.IA) (12, 37), the ribosomal protein L12 (34), and the lipooligosaccharide (19, 33). The adhesins recognize different cellular receptors (29), enabling the pathogen to interact with diverse tissues during the course of infection. Pili are thought to mediate the initial adhesion, followed by a close, primarily Opa-dependent interaction leading to the pathogen's engulfment. The family of the Opa proteins comprises 11 different proteins (2), which are subdivided into two different classes according to their differential binding specificities. The first class, the Opa 50 adhesin, binds to surface heparan sulfate proteoglycan (HSPG) receptors (7, 39). The second class of Opa proteins, the Opa 51-60 adhesins, targets CD66 epitope-containing members of the carcinoembryonic antigen-related cellular adhesion molecules (CEACAMs).Interaction with the different receptor classes induces the ac...