CD34, a type I transmembrane glycoprotein, is a surface antigen which is expressed on several cell types, including hematopoietic progenitors, endothelial cells, as well as mast cells. Recently, CD34 has been described as a marker for epidermal stem cells in mouse hair follicles, and is expressed in outer root sheath cells of the human hair follicle. Although the biological function and regulation of CD34 is not well understood, it is thought to be involved in cell adhesion as well as possibly having a role in signal transduction. In addition, CD34 was shown to be critical for skin tumor development in mice, although the exact mechanism remains unknown.Many proteins' functions and biological activities are regulated through post-translational modifications. The extracellular domain of CD34 is heavily glycosylated but the role of these glycans in CD34 function is unknown. Additionally, two sites of tyrosine phosphorylation have been reported on human CD34 and it is known that CD34 is phosphorylated, at least in part, by protein kinase C; however, the precise location of the sites of phosphorylation has not been reported. In an effort to identify specific phosphorylation sites in CD34 and delineate the possible role of protein kinase C, we undertook the identification of the in vitro sites of phosphorylation on the intracellular domain of mouse CD34 (aa 309-382) following PKC treatment. For this work, we are using a combination of enzymatic proteolysis and peptide sequencing by mass spectrometry. After which the in vivo sites of phosphorylation of full-length mouse CD34 expressed from HEK293F cells were determined. The observed in vivo sites of phosphorylation, however, are not consensus PKC sites, but our data indicate that one of these sites may possibly be phosphorylated by AKT2. These results suggest that other kinases, as well as PKC, may have important signaling functions in CD34.

中文翻译：

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CD34 抗原：体外和体内磷酸化特定位点的测定

CD34 是一种 I 型跨膜糖蛋白，是一种表面抗原，可在多种细胞类型上表达，包括造血祖细胞、内皮细胞以及肥大细胞。最近，CD34 已被描述为小鼠毛囊中表皮干细胞的标记物，并在人类毛囊的外根鞘细胞中表达。尽管 CD34 的生物学功能和调节尚不清楚，但认为它参与细胞粘附以及可能在信号转导中起作用。此外，CD34 被证明对小鼠皮肤肿瘤的发展至关重要，尽管确切的机制仍然未知。许多蛋白质的功能和生物活性是通过翻译后修饰进行调节的。CD34 的胞外域被严重糖基化，但这些聚糖在 CD34 功能中的作用尚不清楚。此外，已经报道了人 CD34 上的两个酪氨酸磷酸化位点，并且已知 CD34 至少部分被蛋白激酶 C 磷酸化；然而，尚未报道磷酸化位点的精确位置。为了鉴定 CD34 中的特定磷酸化位点并描绘蛋白激酶 C 的可能作用，我们在 PKC 处理后对小鼠 CD34（aa 309-382）细胞内结构域的体外磷酸化位点进行了鉴定。对于这项工作，我们结合使用酶促蛋白水解和质谱法肽测序。之后测定了从 HEK293F 细胞表达的全长小鼠 CD34 的体内磷酸化位点。然而，观察到的体内磷酸化位点不是共有的 PKC 位点，但我们的数据表明这些位点之一可能被 AKT2 磷酸化。这些结果表明，其他激酶以及 PKC 可能在 CD34 中具有重要的信号传导功能。