PapG

PapG is the tip adhesin of P fimbriae (pyelonephritis-associated pili) found on many uropathogenic Escherichia coli strains. It is encoded within the pap operon and works in concert with other pilus assembly proteins to mediate attachment of bacteria to host tissues, a key initial step in urinary tract infection.

The PapG adhesin binds to specific glycolipid receptors on uroepithelial cells. Its receptor interactions involve Galα1-4Gal-containing glycosphingolipids, including globoside- or related glycolipids present on urinary tract epithelia. The binding properties of PapG determine tissue tropism and influence the course of infection.

There are three major PapG alleles—PapG I, PapG II, and PapG III—each with distinct receptor-binding preferences. These variants differ in their affinity for particular glycolipid receptors and are associated with different patterns of tissue presence and disease manifestations, contributing to the range of clinical outcomes from cystitis to upper urinary tract infections such as pyelonephritis.

Genetic and regulatory aspects position PapG within the pap operon, alongside structural and assembly genes for P fimbriae. Expression can be modulated by regulatory elements and environmental signals, affecting the level of fimbrial adhesion. Phase variation and regulatory mechanisms influence whether bacteria express P fimbriae and, consequently, their adhesive capabilities in the urinary tract.

In summary, PapG is a key virulence factor that enables uropathogenic E. coli to adhere to urinary tract epithelia via the P fimbrial adhesin, with multiple allelic variants shaping receptor interactions and disease potential.