The Caenorhabditis elegans HAM-1 protein modifies G protein signaling and membrane extension to reverse the polarity of asymmetric cell division

Asymmetric divisions often produce daughter cells that differ in both fate and size. The Caenorhabditis elegans HAM-1 protein regulates both daughter cell fate and daughter cell size asymmetry (DCSA) in a subset of asymmetric divisions. Here we focus on the divisions of the Q.a and Q.p neuroblasts, which use distinct mechanisms to divide with opposite polarity. Q.a divides by a ham-1-dependent, spindle-independent, myosin-dependent mechanism to produce a smaller anterior daughter that dies, whereas Q.p divides by a ham-1-independent, spindle-dependent, myosin-independent mechanism to produce a smaller posterior daughter that dies. Despite these differences, we found that membrane extension at the posterior of Q.a and at the anterior of Q.p promoted DCSA in these cells by a Wiscott-Aldrich protein (WASp)-dependent mechanism and that in ham-1 mutant Q.a divisions, the polarity of this extension was reversed. In addition, the spindle moved posteriorly during the Q.a division in a ham-1 mutant, a phenotype normally exhibited by Q.p. We found that this spindle movement in wild-type Q.p divisions required Ga proteins that promote spindle movement in other asymmetric divisions, and GPR-1, a protein involved in linking G proteins to microtubule asters, localized to the posterior cortex of Q.p. Genetic interactions suggest that ham-1 mutant Q.a divisions also require Ga proteins function to divide with a reversed polarity. The transformation of Q.a to Q.p-like polarity in the ham-1 mutant, however, appeared incomplete: ham-1 loss did not alter the asymmetric localization of the non-muscle myosin NMY-2 to the anterior cortex of Q.a. A GFP tagged ham-1 transgene revealed that Q.a but not Q.p expressed ham-1. Finally, we show that HAM-1 has both cortical and nuclear functions in Q,a DCSA. We propose a model where HAM-1 modifies a default Q.p-type polarity by localizing WASp function to the posterior Q.a membrane and by interfering with G-protein mediated spindle movement. Author Summary One way that animals produce different cell types is by asymmetric cell division, where a cell divides to produce daughter cells that differ in fate. Much is known about the mechanisms that polarize dividing cells to generate daughters that differ in fate. Some asymmetric cell divisions also result in daughters that differ in size, and the mechanisms that regulate how cells generate an asymmetric cleavage plane are poorly understood. In neural progenitors of the nematode Caenorhabditis elegans, two distinct mechanisms generate daughter cells of different size. One type requires movement of the mitotic spindle, which then defines the plane of the cell division. The other is spindle-independent. Here, we study two cells that divide with opposite polarities using these two mechanisms. We find that in the absence of the protein HAM-1, which has been reported to regulate gene transcription, the cell that normally divides using a spindle-independent mechanism now divides with a reversed polarity using a spindle-dependent mechanism. Our findings suggest that HAM-1 plays a key role in defining the mechanism by which a progenitor divides to produce daughter cells of different sizes and that both localization to the cell periphery and nucleus are important for its function.