Pinocytosis as an essential transport mechanism of metazoan cells in culture

Abstract Growth-promoting alpha-globulin (GPAG) has a strong binding capacity for 3 H-thymidine, 3 H-uridine, 3 H-lysine, and 32 PO 4 ‴ and causes an enormous increase in the rate of RNA, DNA, and protein synthesis in metazoan cells. GPAG is taken into the cell by pinocytosis in S and G 2 periods of the cell cycle. Pinosomes travel rapidly towards the perinuclear region before the bound precursors are used in macromolecule biosynthesis. As the GPAG is needed for an actively proliferating culture of metazoan cells in vitro, it is concluded that pinocytosis of GPAG is the controlling factor in deciding whether a cell will continue to grow or will go to the resting state and that the formation of DNA, RNA, and protein may be controlled by the availability of usable precursors in the site of macromolecule biosynthesis. The sources of GPAG in metazoan organism have not been identified but it is reasonable to believe that cell division is regulated primarily by this metazoan factor on an intercellular level.