Modulation of Thr phosphorylation of integrin beta1 during muscle differentiation.
By using transient elevations of cytosolic free calcium levels triggered by integrin antibody or laminin (Kwon, M. S., Park, C. S., Choi, K., Park, C.-S., Ahnn, J., Kim, J. I., Eom, S. H., Kaufman, S. J., and Song, W. K. (2000) Mol. Biol. Cell 11, 1433-1443), we have demonstrated that protein phosphatase 2A (PP2A) is implicated in the regulation of reversible phosphorylation of integrin. In E63 skeletal myoblasts, the treatment of PP2A inhibitors such as okadaic acid and endothall induces an increase of phosphorylation of integrin beta1A and thereby inhibits integrin-induced elevation of cytosolic calcium level and formation of focal adhesions. None of these effects were in differentiated myotubes expressing the alternate beta1D isoform. In the presence of okadaic acid, PP2A in association with integrin beta1A was reduced on myoblasts, whereas beta1D on myotubes remained bound with PP2A. Both co-immunoprecipitation and in vitro phosphatase assays revealed that dephosphorylation of residues Thr788-Thr789 in the integrin beta1A cytoplasmic domain is dependent upon PP2A activity. Mutational analysis of the cytoplasmic domain and confocal microscopy experiments indicated that substitution of Thr788-Thr789 with Asn788-Asn789 is of critical importance for regulating the function of integrin beta1. These results suggest that PP2A may be a primary regulator of threonine phosphorylation of integrin beta1A and subsequent activation of downstream signaling molecules. Taken together, we propose that dephosphorylation of residues Thr788-Thr789 in the cytoplasmic domain of integrin beta1A may contribute to the linkage of integrins to focal adhesion sites and induce the association with cytoskeleton proteins. The switch of integrin beta1A to beta1D isoform in myotubes therefore may be a mechanism to escape from phospho-regulation by PP2A and promotes a more stable association of the cytoskeleton with the extracellular matrix.
Kim
Seon-Myung
SM
Department of Life Science, Kwangju Institute of Science and Technology, Kwangju 500-712, Korea.
Kwon
Min Seong
MS
Park
Chun Shik
CS
Choi
Kyeong-Rock
KR
Chun
Jang-Soo
JS
Ahn
Joohong
J
Song
Woo Keun
WK
D000595:Amino Acid Sequence; D000818:Animals; D002118:Calcium / Q000378:metabolism; D002454:Cell Differentiation; D002478:Cells, Cultured; D003593:Cytoplasm / Q000378:metabolism; D003599:Cytoskeleton / Q000378:metabolism; D003600:Cytosol / Q000378:metabolism; D005109:Extracellular Matrix / Q000378:metabolism; D022001:Focal Adhesions; D019012:Integrin beta1 / Q000378:metabolism*; D018613:Microscopy, Confocal; D008856:Microscopy, Fluorescence; D008969:Molecular Sequence Data; D018482:Muscle, Skeletal / Q000166:cytology / Q000378:metabolism; D009132:Muscles / Q000378:metabolism*; D009154:Mutation; D019319:Okadaic Acid / Q000494:pharmacology; D010749:Phosphoprotein Phosphatases / Q000737:chemistry; D010766:Phosphorylation; D011233:Precipitin Tests; D011485:Protein Binding; D020033:Protein Isoforms; D054648:Protein Phosphatase 2; D017434:Protein Structure, Tertiary; D051381:Rats; D017386:Sequence Homology, Amino Acid; D015398:Signal Transduction; D013912:Threonine / Q000737:chemistry*; D013997:Time Factors
D019012:Integrin beta1; D020033:Protein Isoforms; D019319:Okadaic Acid; D013912:Threonine; D010749:Phosphoprotein Phosphatases; D054648:Protein Phosphatase 2; D002118:Calcium