Tissue-specific expression and regulation by 1,25(OH)2D3 of chick protein kinase inhibitor (PKI) mRNA

Genomic cloning, structure, and regulatory elements of the 1 alpha, 25(OH)2D3 down-regulated gene for cyclic AMP-dependent protein kinase inhibitor

The cyclic AMP-dependent protein kinase inhibitor (PKI) mRNA and protein are negatively and tissue-specifically regulated in the kidney by 1 alpha, 25(OH)2D3. A 17-kb PKI clone, isolated from a chick genomic library, revealed that the PKI gene consists of two exons separated by a 4.5-kb intron. A 411-bp upstream region (constituting 93 bp upstream and 318 bp downstream from the transcriptional start site) containing a putative negative VDRE (nVDRE) fused to the luciferase gene was used for transient transfections of primary cultures of chick kidney cells. Luciferase activity was significantly down-regulated in response to 1 alpha, 25(OH)2D3. This result suggests that the promoter region containing the putative nVDRE plays a pivotal role in the negative regulation of PKI gene transcription.

Binding-dependent disorder-order transition in PKI alpha: a fluorescence anisotropy study

The conformational flexibility of peptidyl ligands may be an essential element of many peptide-macromolecular interactions. Consequently, the alpha-carbonyl backbone flexibility of the 8 kDa protein kinase inhibitor (PKI alpha) peptide of cAMP-dependent protein kinase (cAPK) free in solution and bound to cAPK was assessed by time-resolved fluorescence anisotropy. Specifically, three full-length, single-site PKI alpha mutants (V3C, S28C, and S59C) were prepared, and fluorescein iodoacetamide (FI) was selectively conjugated to the side chains of each substituted cysteine. The time-resolved anisotropy decay profiles of the labeled mutants were well fit to a model-free nonassociative biexponential equation. Free in solution, the three labeled proteins had very similar anisotropy decays arising primarily from local alpha-carbonyl backbone movements. Only a small fraction of the anisotropy decay was associated with slower, whole-body tumbling, confirming that PKI alpha is highly disordered at all three locations. Complexation of the mutants with the catalytic (C) subunit of cAPK decreased the rate of whole-body tumbling for all three mutants. The effects on the rapid decay processes, however, were dependent upon the site of conjugation. The anisotropy decay profiles of both FI-V3C- and FI-S28C-PKI alpha were associated with significantly reduced contributions from the fast decay processes, while that of FI-S59C-PKI alpha was largely unaffected by binding to the C-subunit. The results suggest that the cAPK-binding domain of PKI alpha extends from the its N-terminus to residues beyond Ser28 but does not include the segment around Ser59, which is still part of a highly flexible domain when bound to the C-subunit.