Graduate School of Science, Chiba University, Chiba-shi, Chiba, Japan.
PLoS One. 2012;7(8):e42988. doi: 10.1371/journal.pone.0042988. Epub 2012 Aug 10.
Raine syndrome is caused by mutations in FAM20C, which had been reported to encode a secreted component of bone and teeth. We found that FAM20C encodes a Golgi-localized protein kinase, distantly related to the Golgi-localized kinase Four-jointed. Drosophila also encode a Golgi-localized protein kinase closely related to FAM20C. We show that FAM20C can phosphorylate secreted phosphoproteins, including both Casein and members of the SIBLING protein family, which modulate biomineralization, and we find that FAM20C phosphorylates a biologically active peptide at amino acids essential for inhibition of biomineralization. We also identify autophosphorylation of FAM20C, and characterize parameters of FAM20C's kinase activity, including its Km, pH and cation dependence, and substrate specificity. The biochemical properties of FAM20C match those of an enzymatic activity known as Golgi casein kinase. Introduction of point mutations identified in Raine syndrome patients into recombinant FAM20C impairs its normal localization and kinase activity. Our results identify FAM20C as a kinase for secreted phosphoproteins and establish a biochemical basis for Raine syndrome.
Raine 综合征是由 FAM20C 基因突变引起的，该基因曾被报道编码骨和牙齿的分泌成分。我们发现 FAM20C 编码一种高尔基定位蛋白激酶，与高尔基定位激酶 Four-jointed 有较远的关系。果蝇也编码一种与 FAM20C 密切相关的高尔基定位蛋白激酶。我们表明 FAM20C 可以磷酸化分泌的磷酸化蛋白，包括酪蛋白和 SIBLING 蛋白家族的成员，这些蛋白调节生物矿化，我们发现 FAM20C 在对生物矿化有抑制作用的氨基酸处磷酸化具有生物活性的肽。我们还鉴定了 FAM20C 的自身磷酸化，并表征了 FAM20C 的激酶活性的参数，包括其 Km、pH 和阳离子依赖性以及底物特异性。FAM20C 的生化特性与一种称为高尔基酪蛋白激酶的酶活性相匹配。将在 Raine 综合征患者中发现的点突变引入重组 FAM20C 中会损害其正常定位和激酶活性。我们的结果将 FAM20C 鉴定为分泌磷酸化蛋白的激酶，并为 Raine 综合征建立了生化基础。
