Rac GTPases control axon growth, guidance and branching

Growth, guidance and branching of axons are all essential processes for the precise wiring of the nervous system. Rho family GTPases transduce extracellular signals to regulate the actin cytoskeleton. In particular, Rac has been implicated in axon growth and guidance. Here we analyse the loss-of-function phenotypes of three Rac GTPases in Drosophila mushroom body neurons. We show that progressive loss of combined Rac1, Rac2 and Mtl activity leads first to defects in axon branching, then guidance, and finally growth. Expression of a Rac1 effector domain mutant that does not bind Pak rescues growth, partially rescues guidance, but does not rescue branching defects of Rac mutant neurons. Mosaic analysis reveals both cell autonomous and non-autonomous functions for Rac GTPases, the latter manifesting itself as a strong community effect in axon guidance and branching. These results demonstrate the central role of Rac GTPases in multiple aspects of axon development in vivo, and suggest that axon growth, guidance and branching could be controlled by differential activation of Rac signalling pathways.     

A large nucleolar U3 ribonucleoprotein required for 18S ribosomal RNA biogenesis

Although the U3 small nucleolar RNA (snoRNA), a member of the box C/D class of snoRNAs, was identified with the spliceosomal small nuclear RNAs (snRNAs) over 30 years ago, its function and its associated protein components have remained more elusive. The U3 snoRNA is ubiquitous in eukaryotes and is required for nucleolar processing of pre-18S ribosomal RNA in all organisms where it has been tested. Biochemical and genetic analyses suggest that U3 pre-rRNA base-pairing interactions mediate endonucleolytic pre-rRNA cleavages. Here we have purified a large ribonucleoprotein (RNP) complex from Saccharomyces cerevisiae that contains the U3 snoRNA and 28 proteins. Seventeen new proteins (Utp1 17) and Rrp5 were present, as were ten known components. The Utp proteins are nucleolar and specifically associated with the U3 snoRNA. Depletion of the Utp proteins impedes production of the 18S rRNA, indicating that they are part of the active pre-rRNA processing complex. On the basis of its large size (80S; calculated relative molecular mass of at least 2,200,000) and function, this complex may correspond to the terminal knobs present at the 5' ends of nascent pre-rRNAs. We have termed this large RNP the small subunit (SSU) processome.     

Disruption of dog-1 in Caenorhabditis elegans triggers deletions upstream of guanine-rich DNA

Genetic integrity is crucial to normal cell function, and mutations in genes required for DNA replication and repair underlie various forms of genetic instability and disease, including cancer. One structural feature of intact genomes is runs of homopolymeric dC/dG. Here we describe an unusual mutator phenotype in Caenorhabditis elegans characterized by deletions that start around the 3' end of polyguanine tracts and terminate at variable positions 5' from such tracts. We observed deletions throughout genomic DNA in about half of polyguanine tracts examined, especially those containing 22 or more consecutive guanine nucleotides. The mutator phenotype results from disruption of the predicted gene F33H2.1, which encodes a protein with characteristics of a DEAH helicase and which we have named dog-1 (for deletions of guanine-rich DNA). Nematodes mutated in dog-1 showed germline as well as somatic deletions in genes containing polyguanine tracts, such as vab-1. We propose that DOG-1 is required to resolve the secondary structures of guanine-rich DNA that occasionally form during lagging-strand DNA synthesis.     

Acid-dependent ligand dissociation and recycling of LDL receptor mediated by growth factor homology region

A domain in the low-density lipoprotein receptor contains three cysteine-rich 'growth factor' repeats like those that occur in many proteins. When this domain is deleted, the receptor no longer releases its ligand at acid pH, it is no longer recycled efficiently and it is rapidly degraded after ligand binding.     

Bead movement by single kinesin molecules studied with optical tweezers

Kinesin, a mechanoenzyme that couples ATP hydrolysis to movement along microtubules, is thought to power vesicle transport and other forms of microtubule-based motility. Here, microscopic silica beads were precoated with carrier protein, exposed to low concentrations of kinesin, and individually manipulated with a single-beam gradient-force optical particle trap ('optical tweezers') directly onto microtubules. Optical tweezers greatly improved the efficiency of the bead assay, particularly at the lowest kinesin concentrations (corresponding to approximately 1 molecule per bead). Beads incubated with excess kinesin moved smoothly along a microtubule for many micrometres, but beads carrying from 0.17-3 kinesin molecules per bead, moved, on average, only about 1.4 microns and then spontaneously released from the microtubule. Application of the optical trap directly behind such moving beads often pulled them off the microtubule and back into the centre of the trap. This did not occur when a bead was bound by an AMP.PNP-induced rigor linkage, or when beads were propelled by several kinesin molecules. Our results are consistent with a model in which kinesin detaches briefly from the microtubule during a part of each mechanochemical cycle, rather than a model in which kinesin remains bound at all times.