It is widely accepted that cellular processes are controlled by protein phosphorylation and has become increasingly clear that protein degradation, localization and conformation as well as protein–protein interaction are the examples of subsequent cellular events modulated by protein phosphorylation. Enamel matrix proteins belong to members of the secretory calcium binding phosphoprotein (SCPP) family clustered on chromosome 4q21, and most of the SCPP phosphoproteins have at least one S-X-E motifs (S; serine, X; any amino acid, E; glutamic acid). It has been reported that mutations in C4orf26 gene, located on chromosome 4q21, are associated with autosomal recessive type of Amelogenesis Imperfecta (AI), a hereditary condition that affects enamel formation/mineralization. The enamel phenotype observed in patients with C4orf26 mutations is hypomineralized and partially hypoplastic, indicating that C4orf26 protein may function at both secretory and maturation stages of amelogenesis. The previous in vitro study showed that the synthetic phosphorylated peptide based on C4orf26 protein sequence accelerates hydroxyapatite nucleation. Here we show the molecular cloning of Gm1045, mouse homologue of C4orf26, which has 2 splicing isoforms. Immunohistochemical analysis demonstrated that the immunolocalization of Gm1045 is mainly observed in enamel matrix in vivo. Our report is the first to show that FAM20C, the Golgi casein kinase, phosphorylates C4orf26 and Gm1045 in cell cultures. The extracellular localization of C4orf26/Gm1045 was regulated by FAM20C kinase activity. Thus, our data point out the biological importance of enamel matrix-kinase control of SCPP phosphoproteins and may have a broad impact on the regulation of amelogenesis and AI.

人们普遍认为，细胞过程受蛋白质磷酸化控制，并且越来越清楚的是，蛋白质降解、定位和构象以及蛋白质-蛋白质相互作用是蛋白质磷酸化调节的后续细胞事件的例子。釉质基质蛋白属于聚集在染色体 4q21 上的分泌型钙结合磷蛋白 (SCPP) 家族的成员，大多数 SCPP 磷蛋白具有至少一个 SXE 基序（S；丝氨酸，X；任何氨基酸，E；谷氨酸）。据报道，C4orf26的突变位于染色体 4q21 上的基因与常染色体隐性遗传的牙釉质发育不全 (AI) 相关，这是一种影响牙釉质形成/矿化的遗传性疾病。在C4orf26突变患者中观察到的牙釉质表型是矿化不足和部分发育不全的，表明 C4orf26 蛋白可能在牙釉质形成的分泌和成熟阶段发挥作用。先前的体外研究表明，基于 C4orf26 蛋白序列的合成磷酸化肽可加速羟基磷灰石成核。在这里我们展示了Gm1045的分子克隆，C4orf26的小鼠同源物，它有 2 个剪接异构体。免疫组化分析表明，Gm1045的免疫定位主要在体内的牙釉质基质中观察到。我们的报告首次表明 FAM20C（高尔基酪蛋白激酶）在细胞培养物中磷酸化 C4orf26 和 Gm1045。C4orf26/Gm1045 的细胞外定位受 FAM20C 激酶活性的调节。因此，我们的数据指出了釉质基质激酶控制 SCPP 磷蛋白的生物学重要性，并可能对釉质发生和 AI 的调节产生广泛影响。