Asymmetrically distributed C. elegans homologs of AGS3/PINS control spindle position in the early embryo.
BACKGROUND
Spindle positioning during an asymmetric cell division is of fundamental importance to ensure correct size of daughter cells and segregation of determinants. In the C. elegans embryo, the first spindle is asymmetrically positioned, and this asymmetry is controlled redundantly by two heterotrimeric Galpha subunits, GOA-1 and GPA-16. The Galpha subunits act downstream of the PAR polarity proteins, which control the relative pulling forces acting on the poles. How these heterotrimeric G proteins are regulated and how they control spindle position is still unknown.


RESULTS
Here we show that the Galpha subunits are regulated by a receptor-independent mechanism. RNAi depletion of gpr-1 and gpr-2, homologs of mammalian AGS3 and Drosophila PINS (receptor-independent G protein regulators), results in a phenotype identical to that of embryos depleted of both GPA-16 and GOA-1; the first cleavage is symmetric, but polarity is not affected. The loss of spindle asymmetry after RNAi of gpr-1 and gpr-2 appears to be the result of weakened pulling forces acting on the poles. The GPR protein(s) localize around the cortex of one-cell embryos and are enriched at the posterior. Thus, asymmetric G protein regulation could explain the posterior displacement of the spindle. Posterior enrichment is abolished in the absence of the PAR polarity proteins PAR-2 or PAR-3. In addition, LIN-5, a coiled-coil protein also required for spindle positioning, binds to and is required for cortical association of the GPR protein(s). Finally, we show that the GPR domain of GPR-1 and GPR-2 behaves as a GDP dissociation inhibitor for GOA-1, and its activity is thus similar to that of mammalian AGS3.


CONCLUSIONS
Our results suggest that GPR-1 and/or GPR-2 control an asymmetry in forces exerted on the spindle poles by asymmetrically modulating the activity of the heterotrimeric G protein in response to a signal from the PAR proteins.
Gotta
Monica
M
Wellcome/Cancer Research United Kingdom Institute, Tennis Court Road, University of Cambridge, United Kingdom. monica.gotta@bc.biol.ethz.ch
Dong
Yan
Y
Peterson
Yuri K
YK
Lanier
Stephen M
SM
Ahringer
Julie
J
D000595:Amino Acid Sequence; D000818:Animals; D017173:Caenorhabditis elegans / Q000196:embryology*; D029742:Caenorhabditis elegans Proteins / Q000378:metabolism / Q000502:physiology*; D018797:Cell Cycle Proteins / Q000378:metabolism; D002455:Cell Division / Q000502:physiology*; D016764:Cell Polarity / Q000502:physiology; D017931:DNA Primers; D005455:Fluorescent Antibody Technique; D044385:GTP-Binding Protein alpha Subunits / Q000502:physiology; D013058:Mass Spectrometry; D018715:Microscopy, Video; D008969:Molecular Sequence Data; D017346:Protein Serine-Threonine Kinases; D034622:RNA Interference; D008941:Spindle Apparatus / Q000502:physiology*; D020798:Two-Hybrid System Techniques
D029742:Caenorhabditis elegans Proteins; D018797:Cell Cycle Proteins; D017931:DNA Primers; C475458:G protein regulator 1, C elegans; C475459:G protein regulator 2, C elegans; D044385:GTP-Binding Protein alpha Subunits; C404808:lin-5 protein, C elegans; C097238:PAR-3 protein, C elegans; D017346:Protein Serine-Threonine Kinases; C480838:GPA-16 protein, C elegans