Rbm20 regulates titin alternative splicing as a splicing repressor

Titin, a sarcomeric protein expressed primarily in striated muscles, is responsible for maintaining the structure and biomechanical properties of muscle cells. Cardiac titin undergoes developmental size reduction from 3.7 megadaltons in neonates to primarily 2.97 megadaltons in the adult. This size reduction results from gradually increased exon skipping between exons 50 and 219 of titin mRNA. Our previous study reported that Rbm20 is the splicing factor responsible for this process. In this work, we investigated its molecular mechanism. We demonstrate that Rbm20 mediates exon skipping by binding to titin pre-mRNA to repress the splicing of some regions; the exons/introns in these Rbm20-repressed regions are ultimately skipped. Rbm20 was also found to mediate intron retention and exon shuffling. The two Rbm20 speckles found in nuclei from muscle tissues were identified as aggregates of Rbm20 protein on the partially processed titin pre-mRNAs. Cooperative repression and alternative 3′ splice site selection were found to be used by Rbm20 to skip different subsets of titin exons, and the splicing pathway selected depended on the ratio of Rbm20 to other splicing factors that vary with tissue type and developmental age.     

Cross-regulation between Aurora B and Citron kinase controls midbody architecture in cytokinesis

Cytokinesis culminates in the final separation, or abscission, of the two daughter cells at the end of cell division. Abscission relies on an organelle, the midbody, which forms at the intercellular bridge and is composed of various proteins arranged in a precise stereotypic pattern. The molecular mechanisms controlling midbody organization and function, however, are obscure. Here we show that proper midbody architecture requires cross-regulation between two cell division kinases, Citron kinase (CIT-K) and Aurora B, the kinase component of the chromosomal passenger complex (CPC). CIT-K interacts directly with three CPC components and is required for proper midbody architecture and the orderly arrangement of midbody proteins, including the CPC. In addition, we show that CIT-K promotes Aurora B activity through phosphorylation of the INCENP CPC subunit at the TSS motif. In turn, Aurora B controls CIT-K localization and association with its central spindle partners through phosphorylation of CIT-K''s coiled coil domain. Our results identify, for the first time, a cross-regulatory mechanism between two kinases during cytokinesis, which is crucial for establishing the stereotyped organization of midbody proteins.     

A special repressor/activator system controls distribution of mRNA between translationally active and inactive mRNPs in rabbit reticulocytes

Translation of free mRNPs and polyribosomal mRNPs from rabbit reticulocytes was studied in a rabbit reticulocyte and wheat germ cell-free systems. It has been shown that translation efficiency of polyribosomal mRNPs and the mRNA isolated from the particles is nearly the same in both systems. At the same time, mRNP's translatability, which is high in the homologous cell-free system, is very low in the system from wheat germs. Translation efficiency of free mRNPs in the wheat germ system can be restored by addition of 0.5 M K CI-wash of rabbit reticulocyte ribosomes. The results testify to the existence of some special repressor repressor/activator system which controls the distribution of mRNA between free mRNPs and polyribosomes in rabbit reticulocytes.     

gal4 transcription activator protein of yeast can function as a repressor in Escherichia coli

The chromosomal lac operator of Escherichia coli was replaced by a 22 bp oligonucleotide containing the binding site of the yeast gal4 protein. Induction of gal4 protein synthesis in these bacteria repressed beta-galactosidase synthesis at least 30-fold. These results show that it is possible to detect in bacteria with a simple assay the DNA binding activity of a eukaryotic protein with a defined sequence specificity. This opens new avenues for the isolation in E. coli of mutants of DNA binding proteins unable to bind to their DNA targets, and for direct cloning in bacteria of cDNA coding for DNA binding proteins with defined sequence specificity.     

染色质结构与mRNA可变剪接

mRNA的可变剪接(alternative splicing)是一种由一个mRNA前体(pre-mRNA)通过不同的剪接方式产生多个mRNA变异体(variants)的RNA加工过程。在过去很长一段时间里,人们认为mRNA剪接过程是独立于转录过程的一个转录后RNA加工过程。然而,越来越多的实验证明mRNA剪接在很大程度上是与转录偶联发生的。因此,剪接调控会受到与转录相关因素的调控。本文将对染色质与mRNA剪接调控的相关性和染色质结构调控可变剪接的分子机制进行阐述。