Phosphorylation of phosphophoryn is crucial for its function as a mediator of biomineralization.
Phosphoproteins of the organic matrix of bone and dentin have been implicated as regulators of the nucleation and growth of the inorganic Ca-P crystals of vertebrate bones and teeth. One such protein identified in the dentin matrix is phosphophoryn (PP). It is highly acidic in nature because of a high content of aspartic acid and phosphate groups on serines. The 244-residue carboxyl-terminal domain of rat PP, predominantly containing the aspartic acid-serine repeats, has been cloned, and the corresponding protein has been expressed recombinantly in Escherichia coli. This portion of PP, named DMP2 (dentin matrix protein 2), is not phosphorylated by the bacteria and thus provided a means to study the function of the phosphate groups, the major post-translational modification of native PP. The recombinant DMP2 (rDMP2) possessed much lower calcium binding capacity than native PP. Small angle x-ray scattering experiments demonstrated that PP folds to a compact globular structure upon calcium binding, whereas rDMP2 maintained an unfolded structure. In vitro nucleation experiments showed that PP could nucleate plate-like apatite crystals in pseudophysiological buffer, whereas rDMP2 failed to mediate the transformation of amorphous calcium phosphate to apatite crystals under the same experimental conditions. Collagen binding experiments demonstrated that PP favors the formation of collagen aggregates, whereas in the presence of rDMP2 thin fibrils are formed. Overall these results suggested that the phosphate moieties in phosphophoryn are important for its function as a mediator of dentin biomineralization.
He
Gen
G
Department of Oral Biology, University of Illinois, Chicago, Illinois 60612, USA.
Ramachandran
Amsaveni
A
Dahl
Tom
T
George
Sarah
S
Schultz
David
D
Cookson
David
D
Veis
Arthur
A
George
Anne
A
D000595:Amino Acid Sequence; D000818:Animals; D001224:Aspartic Acid / Q000737:chemistry; D002118:Calcium / Q000378:metabolism; D002850:Chromatography, Gel; D003001:Cloning, Molecular; D024042:Collagen Type I / Q000378:metabolism; D003804:Dentin / Q000378:metabolism; D004926:Escherichia coli / Q000235:genetics; D016326:Extracellular Matrix Proteins; D018625:Microscopy, Atomic Force; D008855:Microscopy, Electron, Scanning; D008903:Minerals / Q000378:metabolism*; D008969:Molecular Sequence Data; D010710:Phosphates / Q000737:chemistry; D010750:Phosphoproteins / Q000737:chemistry / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism* / Q000648:ultrastructure; D010766:Phosphorylation; D017510:Protein Folding; D017434:Protein Structure, Tertiary; D051381:Rats; D017207:Rats, Sprague-Dawley; D011994:Recombinant Proteins / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism; D012694:Serine / Q000737:chemistry; D014074:Tooth Calcification / Q000502:physiology*; D014961:X-Ray Diffraction
D024042:Collagen Type I; D016326:Extracellular Matrix Proteins; D008903:Minerals; D010710:Phosphates; D010750:Phosphoproteins; D011994:Recombinant Proteins; C407548:dentin matrix protein 2; C017630:phosphophoryn; D001224:Aspartic Acid; D012694:Serine; D002118:Calcium