NgCAM takes a scenic route to the axon

Pop goes the acrosome lthough most cell biologists think of molecular motors as chemically driven machines, some of the fastest and most dramatic movements in nature may actually be powered by stored mechanical energy. On page 1183, Shin et al. present a detailed characterization of the forces driving acrosome extension in the sperm of the horseshoe crab Limulus polyphemus , and A A coil of actin (red) springs open in front of the Limulus sperm nucleus (blue). show that this process relies on mechanical energy stored in a molecular spring. Springs also underlie other phenomena such as bacteriophage infection. To penetrate the jelly coat of an egg, Limulus sperm extends a bundle of actin filaments from a coiled position in the sperm head into a sturdy 60-mM-long rod called the acrosomal process. The reaction takes only five seconds. The authors calculated the amount of mechanical energy theoretically required to drive the movement from the energy stored in the structure and expended during extension. Neither ATP hydrolysis nor calcium binding provides enough energy during the reaction, but the potential mechanical energy in the coiled actin bundle is more than sufficient to drive acrosome extension. The data suggest that calcium binding triggers, but does not power, a progressive mechanical uncoiling reaction, extending the acrosomal process like a Jack-in-the-box toy. ᭿ he Drosophila Short stop (Shot/Kakapo) gene encodes several protein isoforms, some of which may link integrins to microtubules. In analyzing the Shot locus, Röper and Brown (page 1305) found something odd: a previously unnoticed exon encoding a series of plakin repeats. The only known function of plakin repeats though is to interact with cytoplasmic intermediate filaments, which flies lack. Based on a biochemical analysis, the plakin repeats are incorporated into a gigantic iso-form of Shot that is the third-largest protein discovered in flies. This isoform includes an actin-binding domain, the plakin repeats, a microtubule-binding domain, and spectrin repeats, and is found in adherens junctions, a localization that seems to be determined by a portion of the plakin domain. Reducing the quantity of the largest Shot isoform in early embryos weakens epithelial intercellular contacts, so it is essential for maintaining epithelial integrity. The authors propose that the giant Shot isoform helps link the adherens junction to an associated belt of actin filaments and microtubules. This novel intermediate filament-independent activity of plakin repeats may be a conserved function of the domain, or it could …