The signal transducer and activator of transcription (STAT) and protein inhibitor of STAT(PIAS) system represent an elegant regulatory mechanism of transcriptional control IN mammalian cytokine signalling. Abnormal activation of the system is associated with immune disorders and a large group of diverse tumours. PIAS3 is a multiple domain protein with distinct functions involved in regulation of cytokine-mediated gene activation pathways.Its over-expression significantly inhibits cell growth and renders cancer cells more sensitive to drugs. The objective of this study was to structurally and biochemically characterise the function of the PIAS3 protein using in silico, in vivo and in vitro analysis approaches.The conservation pattern of the PIAS protein family and critical conserved residues in the PINIT (Proline, Isoleucine, Asparagine, Isoleucine, Tyrosine) domain were identified. The PINIT domain model was generated based on the PINIT domain structure of yeast PIAS3 homologue Siz1 and structural determinants in the PIAS3-STAT3 interaction were evaluated.Guided by the in silico findings, in vivo analysis of the localisation of the PIAS3, mutantderivatives of PIAS3 (PIAS3-L97A, PIAS3-R99N, PIAS3-R99Q), PINIT and acidic domain was conducted. PIAS3 was completely localised in the nucleus while PIAS3 mutants appeared to exhibit diffuse cytoplasmic distribution. The PINIT domain was predominantly localised in the nucleus with some apparent perinuclear staining while the acidic domain exhibited a predominantly perinuclear staining pattern. Further analysis of the PINIT domain and the effect of the mutants on PIAS3-STAT3 interaction were assessed by in vitro analysis. Guided by in silico analysis, the PINIT domain and mutant derivatives of PINIT domain (PINIT-L97A, PINIT-R99N, and PINIT-R99Q) were heterologously expressed in Escherichia coli and subsequently purified using a combination of immobilized metal affinity and size exclusion based chromatography. The size and structural elements of the PINIT domain and its mutants were characterised. The 23 kDa PINIT domain was found to exist as a monomer in solution and its secondary structure was shown to consist of 66 % β-sheets by fourier transformed infrared spectroscopy consistent with the generated homology model.Using surface plasmonresonance spectroscopy (SPR) the PINIT domain was shown to bind to STAT3 in a specific concentration dependent manner. Recombinant PINIT-L97A,PINITR99N and PINIT-R99Q mutants, which exhibited similar structural integrity to the wildtype, were found to abrogate binding to STAT3. These findings suggest that these residues form part of a potential binding surface for stat3. In conclusion, this study has provided evidence that the PINIT domain is an important determinant of PIAS3 interaction with STAT3 and that the interaction is mediated by defined conserved residues directly involved in the PINITSTAT3 interaction.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:rhodes/vital:3991 |
| Date | January 2011 |
| Creators | Mautsa, Nicodemus |
| Publisher | Rhodes University, Faculty of Science, Biochemistry, Microbiology and Biotechnology |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis, Doctoral, PhD |
| Format | 163 leaves, pdf |
| Rights | Mautsa, Nicodemus |
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