Lecture 6 Flashcards
How do lipid soluble hormones work?
- diffuse into the cell through the cell membrane and bind to lipid hormone receptors/txn activators (proteins)
- upon binding, they move into the nucleus, now a tf - bind to specific response elements in genes to regulate gene expression
What are some examples of lipid soluble hormones?
steroid, retinoids, thyroid hormones
What are the three functional regions of a nuclear receptor?
Variable region
DNA-binding domain
Hormone binding domain
What happens to b-galactosidase when the ligand binding domain is intact and the cells are treated with dexamethasone?
b-galactosidase goes to the nucleus
How does a cell send a signal to other cells?
Secreting a lipid soluble hormone
What does the ligand-receptor complex function as?
transcription activator
What is the structure of a transcription factor in a nuclear-receptor superfamily?
- N-terminal region: unique sequence of variable length (100-500 aa)
- Variable region: function as activation domain(s) in most nuclear receptors - DNA binding domain:
- near the center of the receptor primary sequence.
- Contains a repeat of the C4 zinc-finger motif. - Hormone-binding (ligand) domain:
- Near the C-terminal end.
- Contains a hormone-dependent activation domain.
- In some nuclear receptors, the hormone-binding domain functions as a repression domain in the absence of ligand.
What specific receptor is the ligand-binding domain responsible for?
the ligand-binding domain is
responsible for the translocation of in one of the Lipid-Hormone Receptors/Transcription Activators, the Glucocorticoid Receptor (GR), to the nucleus
How does the experiment of fusion proteins work?
Fusion proteins demonstrate that the hormone-binding domain of the glucocorticoid receptor mediates translocation to the nucleus in the presence of hormone.
*Heterodimeric nuclear receptors (e.g., RXR-VDR, RXR-TR, and RXR-RAR): Located exclusively in the nucleus.
*(–) Hormone – repress transcription when bound to their cognate sites in DNA by directing histone deacetylation at nearby nucleosomes
*(+) Hormone – conformational change – binds histone acetylase complexes – reverses repressing effects
*Homodimeric receptors:
*(–) Hormone – in the cytoplasm
*(+) Hormone – receptors translocate to the nucleus and bind to response elements
*Experiment:
*Cultured animal cells – transfected with expression vectors encoding the diagrammed proteins
*Immunofluorescence with a labeled specific anti-β-galactosidase antibody – detects the expressed proteins in transfected cells
*(a) Cells expressing β-galactosidase alone:
*(–) or (+) dexamethasone (Dex) glucocorticoid hormone – enzyme was localized to the cytoplasm
*(b) Cells expressing a fusion protein consisting of β-galactosidase and the entire glucocorticoid receptor (GR) –
*(–) Dex – fusion protein in the cytoplasm
*(+) Dex – fusion protein transported to the nucleus
*(c) Cells expressing a fusion protein composed of β-galactosidase and only the GR ligand-binding domain –
*(–) Dex – fusion protein in the cytoplasm
*(+) Dex – fusion protein transported to the nucleus
*Conclusion – Only the GR domain is required for hormone-dependent receptor transport to the nucleus.
*(d) Model of hormone-dependent gene activation by a homodimeric nuclear receptor:
* (–) Hormone – receptor retained in the cytoplasm by interaction between its ligand-binding domain (LBD) and chaperone proteins
* (+) hormone –
*Hormone complex diffuses through the plasma membrane and binds to the receptor ligand-binding domain.
*Causes a conformational change that releases the chaperone proteins
*Receptor-hormone complex translocates into the nucleus.
*DBD binds to response elements
Ligand-binding domain and an additional activation domain (AD) at the N-terminus stimulate transcription of target genes.
What is the process of the glucocorticoid receptor and translocation of a hormone into the nucleus?
no hormone: the receptor is retained in the cytoplasm by interaction between its ligand-binding domain (LBD) and chaperone proteins
hormone: hormone complex diffuses through the plasma membrane and binds to the receptor-ligand binding domain, this causes a conformational change that releases the chaperone proteins, the receptor-hormone complex translocates into the nucleus and the DBD binds to response elements, the ligand-binding domain and an additional activation domain (AD) at the N-terminus stimulate txn of target genes
What do chaperone proteins do?
interact with transport molecules and transports the protein into the nucleus
What do eukaryotic activators/repressors bind to?
dna elements in promoters or enhancers
What occurs when activators/respressors bind to promoters/enhancers?
affect gene expression by recruiting co-activator/co-repressor complexes
What do co-activator/co-repressor complexes do?
modulate chromatin structure or interact with RNA pol II and GTF’s, they stimulate the assembly of PIC and move nucleosomes away from promoters
What are the two functions of repressor proteins?
May bind to transcription-control elements to inhibit transcription initiation by Pol II
May interact with multiprotein co-repressor complexes to condense chromatin
