Timing of ubiquitin synthesis and conjugation into protein A24 during the HeLa cell cycle

Abstract

To determine the relationship between formation of protein A24 and replication of chromatin, incorporation of 35S methionine into ubiquitin, protein A24, histone 4 and high mobility group proteins (HMG) 1 and 2 were compared to DNA synthesis and cell division in HeLa cells synchronized by thymidine block. The biosynthesis of ubiquitin and its coupling into protein A24 occurred in parrallel and were largely interphase events rising to a maximum rate in early G1 phase and declining to a minimum during cell division. The ratio of labeling of protein A24 to histone 4 indicated that a high rate of ubiquitin-histone 2A conjugation occurred, in excess of that accounted for by histone synthesis. This additional conjugation was highest in early and late G1 phase and its distribution during the cell cycle was similar to the biosynthesis of HMG 1 and 2.