By: Jose Braga
From: Faculdade de Engenharia, Universidade Catolica Portuguesa, and Unidade de Biologia Celular, Instituto de Medicina Molecular, Universidade de Lisboa
At: Complexo Interdisciplinar, Anfiteatro
The ultimate goal of gene expression is the coordinated and controlled production of proteins according to cell requirements. Inside the cellular nucleus of eukaryotes, the amino acid sequence of proteins is encoded in segments of DNA, the genes. Messenger RNAs (mRNAs) carry this information to the ribosomes in the cytoplasm, where proteins are produced. Precursor mRNAs (pre-mRNAs) are formed by the process of transcription from the DNA template. To become fully mature mRNAs, pre-mRNA transcripts undergo a series of processing events. The spliceosome is the large multimolecular machine that catalyses the splicing reaction, a very important pre-mRNA processing step. Spliceosomes have a highly dynamic composition and form anew on nascent pre-mRNAs in each splicing round at the transcription sites. When the mammalian cell nucleus is viewed by light microscopy, spliceosomal components are spread throughout the nucleoplasm, accumulating in several morphologically distinct structures such as the interchromatin granule clusters (IGCs). However transcription of pre-mRNA and, concomitantly, the splicing reaction are known to occur outside these domains. How splicing proteins are recruited to nascent transcripts in the nucleus in order to assemble spliceosomes on newly synthesized pre-mRNAs is unknown. Photobleaching experiments clearly show that spliceosomal proteins move continuously throughout the entire nucleus independently of ongoing transcription or splicing. A mathematical model was then applied to study spliceosome recruitment in the nucleus. The model assumes that splicing proteins move by Brownian motion and interact stochastically with binding sites located at different subnuclear compartments. Simulation of microscopy experiments before and after splicing inhibition yielded results consistent with the experimental observations. Taken together, our data argue against the view that spliceosomal components are stored in IGCs until a signal triggers their recruitment to nascent transcripts. Rather, the results suggest that splicing proteins are constantly diffusing throughout the entire nucleus and collide randomly and transiently with pre-mRNAs.