The Tetrahymena bcd1mutant implicates endosome trafficking in ciliate, cortical pattern formation

Ciliates, such as Tetrahymena thermophila,evolved complex mechanisms to determine both the location and dimensions of cortical organelles such as the oral apparatus (OA: involved in phagocytosis) cytoproct (Cyp: for eliminating wastes) and contractile vacuole pores (CVPs: involved in water expulsion). Mutations have been recovered in Tetrahymenathat affect both the localization of such organelles along anterior-posterior and circumferential body axes, and their dimensions. Here we describe BCD1, a ciliate pattern-gene that encodes a conserved beige-BEACH-domain containing protein a with possible PKA-anchoring activity. Similar proteins have been implicated in endosome trafficking, and are linked to human Chediak-Higashi syndrome and autism. Mutations in the BCD1gene broaden cortical organelle domains as they assemble during pre-division development. The Bcd1 protein localizes to membrane pockets at the base of every cilium that are active in endocytosis. PKA activity has been shown to promote endocytosis in other organisms, so we blocked clathrin-mediated endocytosis (using ‘dynasore’) and inhibited PKA (using H89). In both cases, treatment produced partial phenocopies of the bcd1pattern mutant. This study supports a model in which the dimensions of diverse cortical organelle assembly-platforms may be determined by regulated balance between constitutive exocytic delivery and PKA-regulated endocytic retrieval of organelle materials and determinants.