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Little is known about how metabolism changes during development. For most animal embryos, yolk protein is a principal source of nutrition, particularly of essential amino acids. Within eggs, yolk is stored inside large organelles called yolk platelets (YPs). We have gained insight into embryonic nutrition in the African clawed frog Xenopus laevis by studying YPs. Amphibians follow the ancestral pattern in which all embryonic cells inherit YPs from the eggcytoplasm. These YPs are consumed intracellularly at some point during embryogenesis, but it was not known when, where or how yolk consumption occurs. We have identified the novel yolk protein Seryp by biochemical and mass spectrometric analyses of purified YPs. Within individual YPs, Seryp is degraded to completion earlier than the major yolk proteins, thereby providing a molecular marker for YPs engaged in yolk proteolysis. We demonstrate that yolk proteolysis is a quantal process in which a subset of dormant YPs within embryonic cells are reincorporated into the endocytic system and become terminal degradative compartments. Yolk consumption is amongst the earliest aspects of differentiation. The rate of yolk consumption is also highly tissue specific, suggesting that nutrition in early amphibian embryos is tissue autonomous. But yolk consumption does not appear to be triggered by embryonic cells declining to a critically small size. Frog embryos offer a promising platform for the in vivo analysis of metabolism.
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