A Neurospora crassa Arp1 mutation affecting cytoplasmic dynein and dynactin localization

Of the actin-related proteins, Arp1 is the most similar to conventional act in, and functions solely as a component of the multisubunit complex dynacti n. Dynactin has been identified us an activator of the microtubule-associat ed motor cytoplasmic dynein. The role of Arp1 within dynactin is two-fold: (1) it serves as a structural scaffold protein for other dynactin subunits; and (2) it has been proposed to link dynactin, and thereby dynein, with me mbranous cargo via interaction with spectrin. Using the filamentous fungus Neurospora crassa, we have identified genes encoding subunits of cytoplasmi c dynein and dynactin, In this study, we describe a genetic screen for N. c rassa Arp1 (ro-4) mutants that are defective for dynactin function. We repo rt that the ro-4(EX) mutant is unusual in that it shows alterations in the localization of cytoplasmic dynein and dynactin and in microtubule organiza tion. III the mutant, dynein/dynactin complexes co-localize with bundled mi crotubules at hyphal tips. Given that dynein transports membranous cargo fr om hyphal tips to distal regions, the cytoplasmic dynein and dynactin compl exes that accumulate along microtubule tracts at hyphal tips in the ro-4(E8 ) mutant may have either reduced motor activity or be delayed for activatio n of motor activity following cargo binding.