Bovine activin receptor type IIB messenger ribonucleic acid displays alternative splicing involving a sequence homologous to Src-homology 3 domain binding sites.

Activins are implicated in a variety of biological effects, particularly in reproductive processes such as embryonic development and folliculogenesis. Breakthroughs in the elucidation of the activin signal transduction mechanism were achieved with the characterization of the activin receptors, and the recent identification of cytoplasmic factors apparently involved in the signaling process. The present studies were undertaken to further analyze the activin signaling pathway. The complementary DNA coding for the bovine activin receptor type IIB (bActRIIB) was amplified by RT-PCR from corpus luteum and pituitary RNA, and cloned to characterize its role in activin signal transduction. Two complementary DNA isoforms (bActRIIB2 and bActRIIB5) were detected, coding for 512 amino acids and 498 amino acids, respectively. The shortest isoform lacked a sequence encoding a 14-amino acid stretch very rich in proline residues, located between the transmembrane region and the intracellular kinase domain. Intron sequencing and ribonuclease protection assay demonstrated that alternative splicing is responsible for the generation of these bActRIIB isoforms. This alternative splicing event is unique in that it has not been observed in other species, including the mouse, in which extensive alternative splicing of the ActRIIB messenger RNA is described. Comparison of this alternative sequence with other known proline-rich sequences showed that it has characteristics of a Src-homology 3 domain (SH3) binding site. Coprecipitation experiments have identified two proteins of 69 kDa and 71 kDa from an uterine endometrial cell line, specifically interacting with the short bActRIIB alternative proline-rich sequence. These results suggest that bActRIIB could have a protein-protein interaction, through its putative SH3 binding site, with at least two intracellular SH3-containing proteins.

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