FUNCTIONALLY DISTINCT ISOFORMS OF DYNACTIN ARE EXPRESSED IN HUMAN NEURONS

p150(Glued) is the largest subunit of dynactin, which binds to cytopla smic dynein and activates vesicle transport along microtubules. We hav e isolated human cDNAs encoding p150(Glued) as well as a 135-kDa isofo rm; these isoforms are expressed in human brain by alternative mRNA sp licing of the human DCTN1 gene. The p135 isoform lacks the consensus m icrotubule-binding motif shared by members of the p150(Glued)/Glued/CL IP-170/BIK1 family of microtubule-associated proteins and, therefore, is predicted not to bind directly to microtubules. We used transient t ransfection assays and in vitro microtubule-binding assays to demonstr ate that the p150 isoform binds to microtubules, but the p135 isoform does not. However, both isoforms bind to cytoplasmic dynein, and both partition similarly into cytosolic and membrane cellular fractions. Se quential immunoprecipitations with an isoform-specific antibody for p1 50 followed by a antibody revealed that, in brain, these polypeptides assemble to form distinct complexes, each of which sediments at simila r to 20 S. On the basis of these observations, we hypothesize that the re is a conserved neuronal function for a distinct form of the dynacti n complex that cannot bind directly to cellular microtibules.