CagA interaction with epithelial cells. H. pylori cells with intact cag islands, including an active type IV secretion system, possess a pilus composed of CagY protein. The cagA product is injected into the cytoplasm of the host cell, where tyrosine (Y) residues near its COOH-terminus are phosphorylated. Phosphotyrosine-CagA interacts with several major signal-transduction pathways in the host cell (40, S113), affecting phenotypes including cell morphology, proliferation, and apoptosis (see text). ERK, extracellular signal–regulated kinase; PTPase, protein-tyrosine phosphatase; P, phosphate.