Biosynthesis of granulysin, a novel cytolytic molecule

Granulysin is a newly described lytic molecule expressed by CTL and NK cells. Three mRNA (519, 520, and 522) and two protein products of 15 and 9 kDa are encoded by the granulysin gene. Stable transfectants overexpressing the predominate 520 mRNA were generated to determine the protein products originating from the translation of this message. A transfectant of the NK cell tumor YT overexpressed both 15 and 9 kDa proteins while a transfectant of the T cell tumor HuT78 produced mainly 15 kDa granulysin. Thus the 520 mRNA is sufficient for production of both 15 and 9 kDa granulysin. 9 kDa granulysin accumulated via post-translational processing of 15 kDa protein and was present intracellularly but not in the cell culture supernatant, indicating specific retention of the 9 kDa protein. An inhibitor of granule acidification, concanamycin A, blocked the processing of 15 kDa granulysin to the 9 kDa form. A deduced structural difference between the two forms of the protein and a decrease in lytic activity of 9 kDa granulysin at granule pH suggest two mechanisms by which a granulysin expressing cell is protected from autolysis during the biosynthesis of this potentially harmful molecule.