BMP-15 is a 24-26kD member of the TGF-B superfamily of proteins. It is expressed by late primary follicle oocytes, where it promotes the transition of preantral granulosa cells to cumulus cells, and later the expansion of cumulus cells. Mouse BMP-15 proprecursor is a 50-55kD, 367aa glycoprotein. It is proteolytically cleaved to generate a 40kD prosegment aa26-267 plus a 24kD, 124aa mature region aa268-392 that may be phosphorylated (on Ser7 of the mature molecule) and/or glycosylated. Secreted BMP-15 does not occur as a mature homodimer, but it does exist as a mature monomer, an uncleaved proprecursor, or as a noncovalent heterodimer composed of a cleaved mature region and its prosegment. The heterodimer may also form an oligomer. The BMP-15 prosegment reportedly forms a noncovalent heterodimer with 20kD mature GDF-9. Mature mouse BMP-15 shares 70% and 91% aa identity with human and rat BMP-15.
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