The secretory pathway in Tetrahymena is organized for efficient constitutive secretion at ciliary pockets

In ciliates, membrane cisternae called alveoli interpose between the plasma membrane and the cytoplasm, posing a barrier to endocytic and exocytic membrane trafficking. One exception to this barrier is plasma membrane invaginations called parasomal sacs, which are adjacent to ciliary basal bodies. By following a fluorescent secretory marker called ESCargo, we imaged secretory compartments and secretion in these cells. A cortical endoplasmic reticulum is organized along cytoskeletal ridges and cradles a cohort of mitochondria. One cohort of Golgi are highly mobile in a subcortical layer, while the remainder appear stably positioned at periodic sites close to basal bodies, except near the cell tip where, interestingly, Golgi are more closely spaced. Strikingly, ESCargo secretion was readily visible at positions aligned with basal bodies and parasomal sacs. Thus peri-ciliary zones in ciliates are organized, like ciliary pockets in the highly unrelated trypanosomids, as unique hubs of exo-endocytic trafficking.