Schwarz and Blower reveal that a calcium-regulated ribonuclease promotes the formation of tubular ER networks.
By triggering a wave of intracellular calcium, fertilization induces dramatic changes in the internal organization and protein expression pattern of oocytes. Schwarz and Blower discovered that increased calcium levels activate a ribonuclease called XendoU in Xenopus egg extracts. XendoU doesn’t appear to degrade any specific RNAs in response to calcium. Instead, the researchers found that depleting the ribonuclease delayed nuclear envelope assembly and restricted the formation of tubular ER networks as the extracts exited meiosis. These membrane organization defects could be rescued by the addition of wild-type XendoU but not by catalytically dead versions of the enzyme.
ER membranes are covered with RNAs and associated ribosomes. Schwarz and Blower determined that a portion of XendoU localized to the ER, where it promoted the release of RNA and ribosomal proteins into the cytosol. This activity stimulated the fusion of ER membranes into a dense, tubular network, possibly by giving the membrane fusion machinery a clear space in which to operate.
Knocking down the human homologues of XendoU also altered ER morphology in HeLa cells. Senior author Michael Blower now wants to investigate the ribonuclease’s function in vivo, particularly in tissues where calcium signaling plays a major role, such as in muscles and neurons.
Text by Ben Short