High-mobility group protein HMG-I localizes to G/Q- and C-bands of human and mouse chromosomes

The metaphase specific phosphorylation of HMG I

In vivo labelling of HeLa cells arrested in metaphase with [32P]-phosphate and in vitro phosphorylation of HMG I with the partially purified growth associated H1 kinase was used to study metaphase specific phosphorylation of HMG I. It was found that threonine 53 and 78 became phosphorylated. These amino acids are embedded in respectively the sequence PTPKR and TPGRK which are similar to the sequences phosphorylated by the growth associated H1 kinase.

Structure of the human centromere at metaphase

Until recently the centromere was thought to be a relatively homogeneous region of densely packed heterochromatin with a single differentiated domain--the kinetochore--at its surface, representing the point of attachment of the mitotic spindle. We now know that the centromere of higher eukaryotes is composed of several domains that have been identified using antibody probes. Somewhere within the domains are located both the factor(s) that control the disjunction of sister chromatids and the molecular motor responsible for chromosome movement towards the spindle poles.

Expression of mRNAs encoding mammalian chromosomal proteins HMG-I and HMG-Y during cellular proliferation

The high mobility group chromosomal proteins HMG-I and HMG-Y are closely related isoforms that are expressed at high levels in rapidly dividing, undifferentiated mammalian cells. We analyzed HMG-I/Y mRNA levels at various cell cycle stages in murine NIH/3T3 fibroblasts partially synchronized by seeding from quiescent, contact-inhibited cultures. Flow microfluorometric analysis of DNA content demonstrated a comparable degree of synchronization in such seeded NIH/3T3 cell populations as is obtained by serum deprivation or other means and has the added advantage of avoiding the use of possibly detrimental inhibitors or metabolic starvation to induce such synchrony. We show that HMG-I/Y mRNA levels gradually increase in NIH/3T3 cells during the first 16 h after seeding (G0/G1 to late S phase), but thereafter remain constant, in contrast to the cell cycle-regulated expression of the histone H3 gene. Although there is a 6-fold increase in HMG-I/Y expression during the transition from quiescent to proliferating NIH/3T3 cells, there is a much greater difference in expression (15- to 50-fold) among different cell types, possibly related to their state of differentiation. The HMG-I/Y mRNAs appear to be very stable; there was no decrease in their levels 6 h after actinomycin D transcription termination. The proportion of HMG-I to HMG-Y mRNAs was greater in the human than in the murine cells examined, appeared to be greater in proliferating than in quiescent cells, and did not always correspond with the HMG-I to HMG-Y protein ratio.