Mechanism of externalization of phagocytosis markers in apoptotic cells

凋亡细胞中磷脂酰丝氨酸外化的机制为了阐明膜磷脂磷脂酰丝氨酸（PS）从膜双层的内叶移位到外叶并作为吞噬标记的机制，我们检测了负责凋亡细胞中PS定位的候选酶的量和活性的变化。我们已经克隆了一个编码氨基磷脂移位酶的基因，并获得了其他候选酶，但所有这些酶都与目的酶无关。因此，我们放弃了这个项目。分析候选的新的吞噬标记我们产生了一个单克隆抗体命名为PH 2，抑制巨噬细胞吞噬凋亡细胞。因此，被PH 2识别的抗原被认为是一种新的吞噬标记物。然后，我们对PH 2抗原进行了表征，发现它由蛋白质组成，并定位于正常细胞的膜结构。此外，在trans-Golgi中可检测到抗原的某些组分。这些结果表明，PH 2抗原通过膜囊泡转运，并暴露于细胞表面诱导凋亡。在分析79种人核糖体蛋白中的28种时，发现3种蛋白在凋亡细胞中被降解并从核糖体释放。此外，6个核糖体蛋白在细胞凋亡后可在细胞表面检测到。这些结果表明，在细胞凋亡过程中，一些核糖体蛋白从核糖体移动到细胞表面，并作为吞噬标记物。

Mechanism of phosphatidylserine externalization in apoptotic cellsIn order to elucidate the mechanism by which the membrane phospholipid phosphatidylserine (PS) translocates from the inner to the outer leaflet of the membrane bilayer and serves as a phagocytosis marker, we examined changes in the amount and activity of candidate enzymes responsible for the localization of PS in apoptotic cells. We have cloned a gene coding for presumed amino phospholipid translocase and obtained other candidate enzymes, but all of them turned out to be unrelated to the aimed enzymes. We thus gave up this project.Analysis of candidate novel phagocytosis markerWe generated a monoclonal antibody named PH2 that inhibits macrophage phagocytosis of apoptotic cells. The antigen recognized by PH2 was thus considered to be a novel phagocytosis marker. We then characterized the PH2 antigen and found that it consists of protein and is localized to the membranous structures in normal cells. Furthermore, some fractions of the antigen was detectable in trans-Golgi. These results suggest that the PH2 antigen undergoes membrane vesicle transport and is exposed to cell surface upon apoptosis induction.Externalization of ribosomal proteins in apoptotic callsWe determined structural change of ribosomes during apoptosis. When 28 out of 79 kinds of human ribosomal proteins were analyzed, 3 proteins were found to be degraded and released from ribosomes in apoptotic cells. In addition, 6 ribosomal proteins became detectable on cell surface upon apoptosis. These results suggest that some ribosomal proteins move from the ribosome to cell surface during apoptosis and serve as phagocytosis markers.