Practice question

Cytoplasmic dynein requires adaptor complexes to achieve efficient cargo transport because motor alone exhibits low processivity and weak cargo binding. Dynactin is twenty three subunit complex containing short Arp1 filament, beta-spectrin adaptor, p150Glued subunit with CAP-Gly microtubule-binding domain and coiled-coil dimerization region that binds dynein intermediate chain. Interaction via extended CC1 fragment of p150Glued locks dynein-dynactin together, increasing run length from submicron to several microns by coordinating two motor domains and suppressing detachment. Preventing this interaction by mutating conserved residues in intermediate chain or depleting p50 dynamitin dissociates complex, leaving dynein catalytically active but unable to maintain association with vesicles such as endosomes, phagosomes and mRNA granules. Cellular outcome is reduced retrograde flux, peripheral accumulation, dispersed Golgi and impaired mitotic spindle alignment. Stronger microtubule binding or reversal to plus-end motion does not occur because directionality encoded in AAA ring and linker orientation, not adaptor identity, confirming dynactin primarily as processivity and cargo recruitment factor.