A determinant for directionality of organelle transport in Drosophila embryos.
BACKGROUND
Motor-driven transport along microtubules is a primary cellular mechanism for moving and positioning organelles. Many cargoes move bidirectionally by using both minus and plus end-directed motors. How such cargoes undergo controlled net transport is unresolved.


RESULTS
Using a combination of genetics, molecular biology, and biophysics, we have identified Halo, a novel regulator of lipid droplet transport in early Drosophila embryos. In embryos lacking Halo, net transport of lipid droplets, but not that of other cargoes, is specifically altered; net transport is minus-end directed at developmental stages when it is normally plus-end directed. This reversal is due to an altered balance of motion at the level of individual organelles; without Halo, travel distances and stall forces are reduced for plus-end and increased for minus-end motion. During development, halo mRNA is highly upregulated just as net plus-end transport is initiated (phase II), and its levels drop precipitously shortly before transport becomes minus-end directed (phase III). Exogenously provided Halo prevents the switch to net minus-end transport in phase III in wild-type embryos and induces net plus-end transport during phase II in halo mutant embryos. This mechanism of regulation is likely to be of general importance because the Drosophila genome encodes a family of related proteins with similar sequences, each transiently expressed in distinct domains.


CONCLUSIONS
We conclude that Halo acts as a directionality determinant for embryonic droplet transport and is the first member of a new class of transport regulators.
Gross
Steven P
SP
Department of Developmental and Cell Biology, 2222 Natural Sciences I, University of California-Irvine, Irvine, CA 92697, USA.
Guo
Yi
Y
Martinez
Joel E
JE
Welte
Michael A
MA
D000595:Amino Acid Sequence; D000818:Animals; D001692:Biological Transport / Q000235:genetics / Q000502:physiology; D002874:Chromosome Mapping; D017124:Conserved Sequence / Q000235:genetics; D017931:DNA Primers; D004330:Drosophila; D029721:Drosophila Proteins / Q000235:genetics* / Q000378:metabolism; D018507:Gene Expression Regulation, Developmental; D017403:In Situ Hybridization; D050356:Lipid Metabolism; D008870:Microtubules / Q000378:metabolism*; D008957:Models, Genetic; D020409:Molecular Motor Proteins / Q000378:metabolism*; D008969:Molecular Sequence Data; D015388:Organelles / Q000378:metabolism* / Q000502:physiology; D034622:RNA Interference; D016415:Sequence Alignment
D017931:DNA Primers; D029721:Drosophila Proteins; C486141:Halo protein, Drosophila; D020409:Molecular Motor Proteins