Polarized distribution of mRNAs encoding a putative LDL receptor adaptor protein, xARH (autosomal recessive hypercholesterolemia) in Xenopus oocytes

The Xenopus homologue of hARH (human autosomal recessive hypercholesterolemia) was identified in a screen for vegetally localized RNAs. xARH contains a N-terminal phosphotyrosine binding (PTB) domain that is 91% identical to that of the human gene, a domain previously shown to bind the LDL receptor family members. Maternal xARH, unlike hARH, is present as two transcripts that differ in their 3' UTRs. The large transcript, xARH-alpha, primarily localizes to the oocyte vegetal cortex. The small transcript, xARH-beta, is not localized. During embryogenesis, xARH RNA is found redistributed in a perinuclear pattern. Similar to hARH, xARH is found in the adult liver, but at low levels compared to oocytes. Downstream of the PTB domain is a conserved clathrin box and a C terminal region 50% identical to that of hARH. Previous in vitro studies from this lab have shown xARH can bind the LDLR as well as the vitellogenin (VTG) receptor. We find that injection of the C terminal region missing the PTB domain significantly reduces the internalization of VTG in early stage oocytes, an event that requires the VTG receptor. The data strongly suggest that xARH encodes an adaptor protein that functions in the essential receptor-mediated endocytosis of nutrients during oogenesis. Because xARH protein is found uniformly distributed along the animal/vegetal axis in oocytes, we propose that the localization of xARH-alpha to the vegetal cortex while xARH-beta remains unlocalized, facilitates the uniform distribution of the protein in this extraordinarily large cell.