The gene unc-76 (unc, uncoordinated) is necessary for normal axonal bundling and elongation within axon bundles in the nematode Caenorhabditis elegans. The UNC-76 protein and two human homologs identified as expressed sequence tags are not similar to previously characterized proteins and thus represent a new protein family. At least one of these human homologs can function in C. elegans, suggesting that it, like UNC-76, acts in axonal outgrowth. We propose that the UNC-76 protein, which is found in cell bodies and processes of all neurons throughout development, either has a structural role in the formation and maintenance of axonal bundles or transduces signals to the intracellular machinery that regulates axonal extension and adhesion.Axons in developing nervous systems navigate through a varied set of extracellular environments to reach their targets. Most axons grow along other axons for much of their lengths, and the association of axons in specific bundles, or fascicles, is likely to play a major role in nervous system assembly. Although many cell-surface proteins involved in fascicle formation have been identified (1), relatively little is known about the intracellular mechanisms by which surface interactions lead to the elongation of axons specifically along other axonal surfaces.Genetic screens for fasciculation-defective mutants can, in principle, identify molecules necessary for fasciculation without presuppositions as to their biochemical nature or subcellular localization. Analysis of strains of the nematode Caenorhabditis elegans with mutations causing locomotory defects (uncoordinated or unc mutants) has revealed a group of three genes that, when mutant, affect the growth of axons in fascicles, but not along nonneuronal substrates (cells of the lateral hypodermis and the overlying basement membrane; refs. 2-4). Mutations in this fascicle-specific group of genes, unc-34, unc-71, and unc-76, cause two types of defects: many axons fail to extend fully within the axon bundles of the dorsal and ventral nerve cords, and many fail to remain in their normal fascicles (2-4). The best-characterized example of these defects is provided by the axons of the left and right hermaphrodite-specific neuron (HSN) motor neurons. In fascicle-specific mutants, these axons end prematurely within the left and right fascicles of the ventral nerve cord, respectively, and they often fail to remain on opposite sides of the cord (refs. 3, 4; L.B. and H.R.H., unpublished observations). If a second mutation causes the HSN axons to be rerouted along a lateral process tract instead of the ventral nerve cord, the axons in these mutants grow to their normal lengths, indicating that unc-34 and unc-76 affect the interaction of these axons with the ventral cord environment rather than the ability of the HSN axons to grow beyond a certain length (4).Among the mutants with fascicle-specific defects, unc-76 mutant animals have the most severe abnormalities in locomotion and HSN outgrowth (3, 4). To understand the basis of these...

Fibroblasts, when plated on the extracellular matrix protein fibronectin (FN), rapidly spread and form an organized actin cytoskeleton. This process is known to involve both the central alpha5beta1 integrin-binding and the C-terminal heparin-binding regions of FN. We found that within the heparin-binding region, the information necessary for inducing organization of stress fibers and focal contacts was located in a 29-amino acid segment of FN type III module 13 (III13). We did not find a cytoskeleton-organizing role for repeat III14, which had previously been implicated in this process. Within III13, the same five basic amino acids known to be most important for heparin binding were also necessary for actin organization. A substrate of III13 alone was only weakly adhesive but strongly induced formation of filopodia and lamellipodia. Stress fiber formation required a combination of III13 and III7-11 (which contains the integrin alpha5beta1 recognition site), either as a single fusion protein or as separate polypeptides, and the relative amounts of the two binding sites appeared to determine whether stress fibers or filopodia and lamellipodia were the predominant actin structures formed. We propose that a balance of signals from III13 and from integrins regulates the type of actin structures assembled by the cell.