Spectrin tethers and mesh in the biosynthetic pathway

The parades of how the Golgi and other organelles can sort a continuous flu x of protein and lipid but maintain temporal and morphological stability re mains unresolved, Recent discoveries highlight a role for the cytoskeleton in guiding the structure and dynamics of organelles. Perhaps one of the mor e striking, albeit less expected, of these discoveries is the recognition t hat a spectrin skeleton associates with many organelles and contributes to the maintenance of Golgi structure and the efficiency of protein traffickin g in the early secretory pathway, Spectrin interacts directly with phosphoi nositides and with membrane proteins. The small GTPase ARF, a key player in Golgi dynamics, regulates the assembly of the Golgi spectrin skeleton thro ugh its ability to control phosphoinositide levels in Golgi membranes, wher eas adapter molecules such as ankyrin link spectrin to other membrane prote ins. Direct interactions of spectrin with actin and centractin (ARP1) provi de a link to dynein, myosin and presumably other motors involved with intra cellular transport. Building on the recognized ability of spectrin to organ ize macromolecular complexes of membrane and cytosolic proteins into a mult ifaceted scaffold linked to filamentous structural elements (termed linked mosaics), recent evidence supports a similar role for spectrin in organelle function and the secretory pathway. Two working models accommodate much of the available data: the Golgi mesh hypothesis and the spectrin ankyrin ada pter protein tethering system (SAATS) hypothesis.