Rab27a is required for regulated secretion in cytotoxic T lymphocytes

Rab27a activity is affected in several mouse models of human disease includ ing Griscelli (ashen mice) and Hermansky-Pudlak (gunmetal mice) syndromes. A loss of function mutation occurs in the Rab27a gene in ashen (ash), where as in gunmetal (gm) Rab27a dysfunction is secondary to a mutation in the oc subunit of Rab geranylgeranyl transferase, an enzyme required for prenylat ion and activation of Rabs. We show here that Rab27a is normally expressed in cytotoxic T lymphocytes (CTLs), but absent in ashen homozygotes (ash/ash ). Cytotoxicity and secretion assays show that ash/ash CTLs are unable to k ill target cells or to secrete granzyme A and hexosaminidase. By immunofluo rescence and electron microscopy, we show polarization but no membrane dock ing of ash/ash lytic granules at the immunological synapse. In gunmetal CTL s, we show underprenylation and redistribution of Rab27a to the cytosol, im plying reduced activity. Gunmetal CTLs show a reduced ability to kill targe t cells but retain the ability to secrete hexosaminidase and granzyme A. Ho wever, only some of the granules polarize to the immunological synapse, and many remain dispersed around the periphery of the CTLs. These results demo nstrate that Rab27a is required in a final secretory step and that other Ra b proteins also affected in gunmetal are likely to be involved in polarizat ion of the granules to the immunological synapse.