The flagellum and flagellar pocket of trypanosomatids.

The flagellum and flagellar pocket are distinctive organelles present among all of the trypanosomatid protozoa. Currently, recognized functions for these organelles include generation of motility for the flagellum and dedicated secretory and endocytic activities for the flagellar pocket. The flagellar and flagellar pocket membranes have long been recognized as morphologically separate domains that are component parts of the plasma membrane that surrounds the entire cell. The structural and functional specialization of these two membranes has now been underscored by the identification of multiple proteins that are targeted selectively to each of these domains, and non-membrane proteins have also been identified that are targeted to the internal lumina of these organelles. Investigations on the functions of these organelle-specific proteins should continue to shed light on the unique biological activities of the flagellum and flagellar pocket. In addition, work has begun on identifying signals or modifications of these proteins that direct their targeting to the correct subcellular location. Future endeavors should further refine our knowledge of targeting signals and begin to dissect the molecular machinery involved in transporting and retaining each polypeptide at its designated cellular address.