Characterization of a novel membrane complex required for docking and regulated exocytosis of lysosome-related organelles in Tetrahymena thermophila

In the ciliate Tetrahymena thermophila, lysosome-related organelles called mucocysts accumulate at the cell periphery where they secrete their contents in response to extracellular events, a phenomenon termed regulated exocytosis. A forward genetic approach uncovered MDL1 (Mucocyst Discharge with a LamG domain) as essential gene for regulated exocytosis of mucocysts. The encoded protein, Mdl1p, is a 66 kD membrane-associated glycoprotein that localizes to mucocysts, and specifically to a tip domain that contacts the plasma membrane when the mucocyst is docked. Immunoisolation by affinity capture using Mdl1p-FLAG as bait, described here, revealed that Mdl1p forms a complex with 9 other proteins, all of them novel and largely restricted to Alveolates.