Granulysin, a T cell product, kills bacteria by altering membrane permeability

Granulysin, a protein located in the acidic granules of human NK cells and cytotoxic T cells, has antimicrobial activity against a broad spectrum of m icrobial pathogens. A predicted model generated from the nuclear magnetic r esonance structure of a related protein, NK lysin, suggested that granulysi n contains a four alpha helical bundle motif, with the alpha helices enrich ed for positively charged amino acids, including arginine and lysine residu es, Denaturation of the polypeptide reduced the a helical content from 49 t o 18% resulted in complete inhibition of antimicrobial activity. Chemical m odification of the arginine, but not the lysine, residues also blocked the antimicrobial activity and interfered with the ability of granulysin to adh ere to Escherichia coil and Mycobacterium tuberculosis. Granulysin increase d the permeability of bacterial membranes, as judged by its ability to allo w access of cytosolic beta -galactosidase to its impermeant substrate. By e lectron microscopy, granulysin triggered fluid accumulation in the periplas m of M, tuberculosis, consistent with osmotic perturbation, These data sugg est that the ability of granulysin to kill microbial pathogens is dependent on direct interaction with the microbial cell wall and/or membrane, leadin g to increased permeability and lysis.