Differential expression of VegT and Antipodean protein isoforms in Xenopus

The VegT/Antipodean (Apod) gene is important for germ layer formation in Xenopus. To investigate the role of this gene at the protein level, as opposed to the RNA level, we have generated affinity purified polyclonal antibodies to Apod, and for comparison, to the other early T-box proteins Xbrachyury and Eomesodermin. An anti-VegT/Apod antibody reveals that there are two protein isoforms in Xenopus, one that we refer to as VegT and a smaller molecular weight isoform that we refer to as Apod. These isoforms have different N-terminal domains resulting from developmentally regulated alternative splicing of a primary transcript arising from a single VegT/Apod gene. VegT is maternally expressed. Its translation is blocked during oogenesis but the protein is present from the egg until gastrulation in the presumptive endoderm. There is no evidence for zygotic expression of this isoform. Conversely, the Apod protein isoform is expressed only after the onset of zygotic transcription in the presumptive mesoderm and is inducible by activin. We conclude that the developmental role of VegT/Apod is mediated by two different proteins, with entirely different patterns of expression and response to growth factors.