However, the mechanism of how KIF4 and condensins localize to the Chromosome Scaffold is poorly understood. Kinesin family member 4 (KIF4) and condensins I and II are essential chromosomal proteins for Chromosome organization by locating primarily to the Chromosome Scaffold. Collectively, our novel results revealed that Cdk1-dependent KIF4A phosphorylation at S1186 is a trigger for chromosomal organization during early mitosis. Although both KIF4A and condensin I disappeared from the Chromosomes, the chromosomal localization of condensin II was not affected. Additionally, a defect in Chromosome segregation, Chromosome bridge formation, was often observed. As a result, the morphology of the Chromosomes was observed to be laterally decondensed, without condensin I in the Chromosome Scaffold. In addition, defects in KIF4A phosphorylation were found to disrupt the interaction of KIF4A with the condensin I complex. The KIF4A mutant, which is not phosphorylated at S1186, was found to localize to the nucleus during interphase but did not accumulate in the Chromosome Scaffold after nuclear envelope breakdown. Here, we demonstrate that Cdk1-dependent phosphorylation of KIF4A at S1186 is required for Chromosome binding and Chromosome Scaffold formation. However, the relationship between the function of KIF4A and its phosphorylation remains unclear. Kinesin family member 4A (KIF4A) plays an important role in the Chromosome organization through the formation of the Chromosome Scaffold structure. Using the locations of gold nanoparticles to visualize the underlying structure, the tomograms we obtained reveal the patterns of Chromosome Scaffold organization, which appears to consist of a helical structure that serves to organize chromatin loops into the metaphase Chromosome.Ĭhromosome organization during cell division is achieved through the timely association of proteins with chromatin and is regulated by protein phosphorylation. The Chromosome was stained with immunogold-labeled condensin complex, one of the major Chromosome Scaffold proteins and then observed in three dimensions using ET. Here we present a new technique that enables the observation of the Chromosome Scaffold structure in metaphase Chromosomes from any direction, by transferring an isolated Chromosome to a 360° rotational holder for electron tomography (ET). However, it remains to be elucidated how the Chromosome Scaffold organizes the mitotic Chromosome and how it supports shaping the structure of the Chromosome during metaphase. It plays a vital role in Chromosome condensation, shaping the X-shaped structure of the mitotic Chromosome, and also provides flexibility for Chromosome movement during cell division. Abstract The Chromosome Scaffold is considered to be a key structure of the mitotic Chromosome.
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