C201R, with a point mutation that leads to a non-conservative substitution within GARS. Heterozygous mice with a C3H genetic background have loss of grip strength, decreased motor flexibility and disruption of fine motor control; this relatively mild phenotype is more severe on a C57BL/6 background. Homozygous mutants have a highly deleterious set of features, including movement difficulties and death before weaning. Heterozygous animals have a reduction in axon diameter in peripheral nerves, slowing of nerve conduction and an alteration in the recovery cycle of myelinated axons, as well as innervation defects. An assessment of GARS levels showed increased protein in 15-day-old mice compared with controls; however, this increase was not observed in 3-month-old animals, indicating that GARS function may be more crucial in younger animals. We found that enzyme activity was not reduced detectably in heterozygotes at any age, but was diminished greatly in homozygous mice compared with controls; thus, homozygous animals may suffer from a partial loss of function. The Gars C201R mutation described here is a contribution to our understanding of the mechanism by which mutations in tRNA synthetases, which are fundamentally important, ubiquitously expressed enzymes, cause axonopathy in specific sets of neurons.

Sorting nexins (Snxs) are a recently discovered family of conserved hydrophilic cytoplasmic proteins that have been found associated with membranes of the endocytic system and that are implicated in the trafficking of many endosomal membrane proteins, including the epidermal growth factor receptor and transferrin receptor. Snx proteins are partly defined by the presence of a p40 phox homology domain that has recently been shown to bind phosphatidylinositol 3-phosphate. Most Snx proteins also contain a predicted coiled-coils domain in the carboxyl-terminal half of the protein and have been shown to form dimers with other members of the Snx family. The yeast sorting nexins Vps5p and Vps17p form a dimer and are also components of the retromer complex that mediates endosome-to-Golgi transport of the carboxypeptidase Y receptor Vps10p. To functionally define the different domains of the yeast sorting nexins Vps5p and Vps17p, we have generated various truncations to examine the role that the different domains of Vps5p/ Vps17p play in their respective functions. Herein, we show that the C-terminal halves of Vps5p and Vps17p, which contain the coiled-coils domains, are necessary and sufficient for their interaction. We have also mapped the retromer assembly domain to the N-terminal half of Vps5p and found that binding of Vps5p by Vps17p synergizes the interaction between Vps5p and other retromer components. Additionally, we have examined which domain(s) of Vps5p is necessary for membrane association.

We report here an in vivo study of kinesin heavy chain (KHC) functions in yeast. We have identified in Schizosaccharomyces pombe a kinesin motor gene, klp3+, which has the highest homology to the Neurospora crassa KHC. Using indirect immunofluorescence, HA epitope‐tagged Klp3 protein is cytoplasmic and appears as one to a few distinct patches that are coincident with microtubules. The klp3 null allele is viable. In klp3 deleted cells, ER, Golgi and mitochondrial distribution appear normal. Mitochondrial distribution in S. pombe is known to be microtubule‐associated. We show that latrunculin A does not cause mitochondria to aggregate, suggesting that mitochondrial distribution in fission yeast, unlike budding yeast, is not dependent upon actin‐based processes. Neither latrunculin A nor thiabendazole affects ER or Golgi distribution. We also used the vital dye FM4‐64 to visualize the internalization of the dye and its transport to vacuoles in fission yeast in the presence and absence of Klp3. We observed no significant difference between the wild‐type and Klp3 null cells in either the dynamics of endocytosis or the distribution and fusion of vacuoles. The drug brefeldin A causes Golgi‐to‐ER recycling in wild‐type fission yeast cells. Although recycling of Golgi to ER after brefeldin A treatment occurs in klp3 null cells, recycling is defective and the distribution pattern we see is different from that observed in the wild‐type strain. We conclude that Klp3 plays a role in BFA‐induced membrane transport. The nucleotide sequence of S. pombe klp3+ was submitted to GenBank under Accession No. AF154055. Copyright © 2000 John Wiley & Sons, Ltd.