BS2550 - Neuronal and Cellular Signalling Lecture 14 Nuclear Function The nucleus has a double membrane which contains pores that regulates transport into the nucleus. Proteins imported into the nuclei have specific sequences known as nuclear localisation signals (NLS). Nuclear pores are composed of many proteins called nucleoporins. - They allow ions and small proteins to diffuse freely through the pores. Nuclear localisation signals or cell stimulation can lead to the localisation of proteins into the nucleus. Stimulation of cells can lead to localisation of proteins into the nucleus. A range of second messenger-dependent pathways lead to movement of enzymes into the nucleus. Kinase activity alters nuclear activity and altered gene transcription causes new protein synthesis to control cell functions. CAMP Response Element (CREB) This is a typical signal transduction mechanism leading to transcription control. The signal activates the receptor at the cell surface. This produces a second messenger (CAMP). This activates protein kinaseff the kinase translocates to the nucleus. This phosphorylates CREBff pCREB binds to the CTE region on the promoter. Activated CREB binding protein (CBP) with histone acetyltransferase (HAT) activity changes gene transcription. This process is critical in memory and learning. DNA Humans have a lot of DNAff over 3 million base pairs. But only about 1.5% of human DNA is of a known function (coding/RNA/regulatory). Multiple histones wrap into a 30nm fibre consisting of nucleosome arrays in their most compact form (heterochromatin). Chromatin structure: - Inactive genes are organised into regions of condensed chromatin. - The compact nature of chromatin containing nucleosomes enables storage of large amounts of DNA in a cell.
- A drawback of this storage is the inaccessibility of proteins to promoters for transcription. - Upstream activator sites provide regions where sequence specific transcription factors bind. - Transcription factors facilitate co-activators binding to DNA and the regulation of transcription. Histones are found in all eukaryotic cells. DNA Nucleosomes comprise of DNA wrapped around histone proteins. H2A H2B There are different types of histone proteins - H1ff H2Aff H2Bff H3. H4. H4 H3 H1 Nucleosome The nucleosome contains 8 core histones (H2Aff H2Bff H3 and H4) and one liner histone. Core of 8 Histones The eight core histones in a nucleosome comprises of 2 of each of the core histones. Each nucelosome wraps 147 base pairs around the core 1.65 times. H1 (linker histone) binds the DNA at the start and finish of core binding. Histone cores have a tail for modification. Histone modification: - Regular arrays of histones are packed together to make filaments and fibres. - Each histone has two basic parts: o C terminal - highly conservedff DNA binding. o N terminal - acts as a site for covalent modification. - Multiple types of modification of the N terminal alters the structure of packed DNA. Acetylation uses acetyltransferases and deacetylases and occurs on lysine. Methylation occurs on lysine and arginine. Phosphorylation occurs on serine/threonine and histidine. Ubiquitination is a small protein that can be added to DNA. Histone modifications changes have distinct