Rala and the exocyst control Pvr trafficking and signaling to ensure lymph gland homeostasis in Drosophila melanogaster

The balance between hematopoietic progenitors and differentiated hemocytes is finely tuned during development. In the larval hematopoietic organ of Drosophila, called the lymph gland, the receptor tyrosine kinase Pvr signals from differentiated cells to maintain a pool of undifferentiated progenitors. However, little is known about the processes that support Pvr function. The small GTPase Ral is involved in the regulation of several membrane trafficking events. Drosophila has a single Ral protein, Rala, which has been implicated in the development of various tissues. Here, we investigated the involvement of Rala in the larval fly hematopoietic system. We discovered that the loss of Rala activity phenocopies Pvr loss of function by promoting hemocyte progenitor differentiation. Moreover, using epistasis analysis, we found that the guanine exchange factor RalGPS lies upstream of Rala in this event, whereas the exocyst and Rab11 are acting downstream. Strikingly, the loss of Rala activity leads to a considerable accumulation of Pvr at the plasma membrane, hence suggesting a trafficking defect and reduced Pvr function. Consistent with this hypothesis, Rala loss of function phenotype in the lymph gland is fully suppressed by constitutive STAT activity, which normally mediates Pvr function in the lymph gland. Together, our findings unravel a novel RalGPS-Rala-exocyst-Rab11 axis for the maintenance of lymph gland homeostasis through Pvr.