Self-regulated polymerization of the actin-related protein Arp1

The actin-related protein Arp1 (or centractin, actin RPV) is the major subu nit of dynactin, a key component of the cytoplasmic dynein motor machinery [1-3]. Of the ubiquitously expressed members of the Arp superfamily, Arp1 i s most similar to conventional actin [4-6] and, on the basis of conserved s equence features, is predicted to bind ATP and possibly polymerize. In vivo , all cytosolic Arp1 sediments at 20S [7] suggesting that it assembles into oligomers, most likely dynactin - a multiprotein complex known to contain eight or nine Arp1 monomers in a 37 nm filament [8]. The uniform length of Arp1 polymers suggests a novel assembly mechanism that may be governed by a 'ruler' activity. In dynactin, the Arp1 filament is bounded by actin-cappi ng protein at one end and a heterotetrameric protein complex containing the p62 subunit (D.M. Eckley, S.R. Gill, J.B.B., J.E. Heuser, T.A.S., unpublis hed observations) at the other [8]. In the present study, we analyzed the b ehavior of highly purified, native Arp1.Arp1 was found to polymerize rapidl y into short filaments that were similar, but not identical, in length to t hose in dynactin. With time, these filaments appeared to anneal to form lon ger assemblies but never attained the length of conventional actin filament s.