Microtubules and Intracellular Traffic of Secretory Proteins in Rat Extraorbital Lacrimal Glands

Abstract

Proteins destined to be secreted via the regulated pathway are synthesized in the endoplasmic reticulum (ER), transit through the various cisternae of the Golgi apparatus where they become modified, and finally are stored in secretory granules. Under stimulation of the cells, secretory granules fuse with the apical plasma membrane, and their content is released into the extracellular medium. It is now well established that every step of the transport occurring between these different compartments involves vesicular carriers,¹ but the role of the cytoskeleton in the secretory pathway is still under discussion. In many cell types, the rate of the intracellular traffic of proteins is reduced in the presence of microtubule depolymerizing agents, so microtubules were first thought to play a role in the movement of the secretory vesicles.² Recent in vitro results have shown that vesicle budding, movement, and fusion do not require the cytoskeleton, so the microtubules are now considered mainly to play a role in the maintenance of a correct location of the organelles rather than in a direct guiding of the secretory vesicles.³