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To study Xenopus insulin-like growth factor I (IGF-I) receptor messenger expression during oogenesis, we isolated a complementary DNA (cDNA) corresponding to the beta-subunit of the receptor. There is a high degree of conservation between the deduced polypeptide and the three mammalian sequences previously described for the IGF-I receptor (75% homology) even though it is lower than the homology among mammals themselves (95% homology). IGF-I receptor messenger RNAs were specifically detected by reverse transcription-PCR in oocytes, embryos, and adult liver and muscle. By in situ hybridization, these messenger RNAs could be visualized only in oocytes. Quantification showed that they accumulated from the previtellogenic stage until early vitellogenesis. No specific labeling could be detected in oocytes after stage IV of vitellogenesis. Thus, the IGF-I receptor messenger stock does not seem to increase further beyond this point or may even decrease. The long 3'-untranslated sequence (1.8 kilobases) included in the cDNA presents no homology with those of mammalian receptor cDNAs and could be longer, as no polyadenylated tail is observed. Some motifs corresponding to sequence described as cytoplasmic polyadenylation element or that have been described in unstable messengers could be observed. Moreover, a deadenylation of this RNA occurs in the postvitellogenic stage. These results suggest that translation occurred very early during oogenesis. Therefore, IGF-I receptor could play a role early on, during oocyte growth, in addition to its involvement in the maturation process.
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