Nuclear variants of bone morphogenetic proteins.
BACKGROUND
Bone morphogenetic proteins (BMPs) contribute to many different aspects of development including mesoderm formation, heart development, neurogenesis, skeletal development, and axis formation. They have previously been recognized only as secreted growth factors, but the present study detected Bmp2, Bmp4, and Gdf5/CDMP1 in the nuclei of cultured cells using immunocytochemistry and immunoblotting of nuclear extracts.


RESULTS
In all three proteins, a bipartite nuclear localization signal (NLS) was found to overlap the site at which the proproteins are cleaved to release the mature growth factors from the propeptides. Mutational analyses indicated that the nuclear variants of these three proteins are produced by initiating translation from downstream alternative start codons. The resulting proteins lack N-terminal signal peptides and are therefore translated in the cytoplasm rather than the endoplasmic reticulum, thus avoiding proteolytic processing in the secretory pathway. Instead, the uncleaved proteins (designated nBmp2, nBmp4, and nGdf5) containing the intact NLSs are translocated to the nucleus. Immunostaining of endogenous nBmp2 in cultured cells demonstrated that the amount of nBmp2 as well as its nuclear/cytoplasmic distribution differs between cells that are in M-phase versus other phases of the cell cycle.


CONCLUSIONS
The observation that nBmp2 localization varies throughout the cell cycle, as well as the conservation of a nuclear localization mechanism among three different BMP family members, suggests that these novel nuclear variants of BMP family proteins play an important functional role in the cell.
Felin
Jenny E
JE
Department of Microbiology and Molecular Biology, Brigham Young University, Provo, Utah, USA.
Mayo
Jaime L
JL
Loos
Trina J
TJ
Jensen
J Daniel
JD
Sperry
Daniel K
DK
Gaufin
Stephanie L
SL
Meinhart
Christopher A
CA
Moss
Jennie B
JB
Bridgewater
Laura C
LC
D000595:Amino Acid Sequence; D000818:Animals; D055396:Bone Morphogenetic Protein 2 / Q000032:analysis / Q000235:genetics / Q000378:metabolism*; D055415:Bone Morphogenetic Protein 4 / Q000032:analysis / Q000235:genetics / Q000378:metabolism*; D002455:Cell Division; D002460:Cell Line; D002467:Cell Nucleus / Q000378:metabolism*; D018387:Codon, Initiator; D055428:Growth Differentiation Factor 5 / Q000032:analysis / Q000378:metabolism; D051379:Mice; D008969:Molecular Sequence Data; D015398:Signal Transduction
D055396:Bone Morphogenetic Protein 2; D055415:Bone Morphogenetic Protein 4; D018387:Codon, Initiator; C525894:Gdf5 protein, mouse; D055428:Growth Differentiation Factor 5