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Abstract
In mitosis, chromosomes achieve their characteristic shape through condensation, an essential process for proper segregation of the genome during cell division. A classical model for mitotic chromosome condensation proposes that non-histone proteins act as a structural framework called the chromosome scaffold. The components of the chromosome scaffold, such as DNA topoisomerase IIα (TOP2A) and structural maintenance of chromosomes protein 2 (SMC2), are necessary to generate stable mitotic chromosomes; however, the existence of this scaffold remains controversial. The aim of this study was to determine the protein composition of the chromosome scaffold. We used the DT40 chicken cell line to isolate mitotic chromosomes and extract the associated protein fraction, which could contain the chromosome scaffold. MS revealed a novel component of the chromosome scaffold, bromodomain adjacent to zinc finger 1B (BAZ1B), which was localized to the mitotic chromosome axis. Knocking out BAZ1B caused prophase delay because of altered chromosome condensation timing and mitosis progression errors, and the effect was aggravated if BAZ1A, a BAZ1B homolog, was simultaneously knocked out; however, protein composition of prometaphase chromosomes was normal. Our results suggest that BAZ1 proteins are essential for timely chromosome condensation at mitosis entry. Further characterization of the functional role of BAZ1 proteins would provide new insights into the timing of chromosome condensation.
Highlights
In mitosis, chromosomes achieve their characteristic shape through condensation, an essential process for proper segregation of the genome during cell division
It was shown that mitotic chromosome scaffold proteins such as ScI (TOP2A) and ScII (SMC2) were accumulated in the fraction of histone-depleted mitotic chromosomes [29] and that the addition of 1% -mercaptoethanol dissociated the chromosome scaffold (Fig. 1A) [2, 29]
To obtain accurate results, we compared proteins in the isolated scaffold with those in the background obtained by chromosome scaffold isolation with 1% -mercaptoethanol
Summary
Chromosomes achieve their characteristic shape through condensation, an essential process for proper segregation of the genome during cell division. A classical model for mitotic chromosome condensation proposes that non-histone proteins act as a structural framework called the chromosome scaffold. A classical model proposes that non-histone proteins act as a structural framework during the formation of mitotic chromosomes [1] This “chromosome scaffold” was hypothesized to be an insoluble biochemical fraction that could be obtained from chromosomes after most of the DNA and proteins were solubilized [2]. The major components of this fraction are structural maintenance of chromosomes protein 2 (SMC2) and DNA topoisomerase II␣ (TOP2A) [3], which is thought to affect chromosome condensation during late mitosis [4, 5].
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