'American Society for Biochemistry & Molecular Biology (ASBMB)'
Abstract
Heterotrimeric G-proteins function as signal transducers for a variety of hormone-coupled enzyme and ion transport systems in eukaryotic cells. We have studied G-protein-coupled processes that appear to be developmentally regulated in polarized pig kidney cells (LLC-PK1). Following trypsinization, LLC-PK1 cells differentiate from a rounded cell type to a fully polarized epithelium by 7 days of culture. During this differentiation, the expression of G-protein alpha-i-2 subunit mRNA was not detected until day 4 of culture, it peaked at day 6, and declined thereafter. In contrast, G-protein alpha-(s) subunit mRNA which peaked on day 4 was easily detected on all culture days. The presence of the alpha-i-2 protein on epithelial cell basolateral membranes followed the same pattern of mRNA expression during culture. To understand the developmental expression of the alpha-i-2 subunit in non-polarized cells and its potential regulation by hormones and second messengers in polarized cells at the transcriptional level, genomic DNA segments encoding the alpha-i-2 gene promoter were isolated from an EMBL-3 porcine genomic library. S1 nuclease analysis of LLC-PK1 mRNA with cRNA probes derived from these DNA segments revealed major and a minor transcriptional start sites 131 and 171 base pairs upstream of the translation initiation site. The porcine and human alpha-i-2 subunit genes shared a 78% sequence identity in their 5' flanks which suggested an evolutionary conversation of cis elements required to influence their transcription. The porcine alpha-i-2 gene promoter was identified by fusing DNA segments encoding putative 5'-flanking areas of the gene to a plasmid that contained a firefly luciferase reporter gene but lacked a promoter. The minimal promoter was found between -130 and -60 base pairs from the major transcription start site. No typical "TATA-like" sequences were found. However, a "GC" box and a "TGTGG" sequence were two potential cis elements required for basal transcription of the porcine gene promoter which shared a 76% sequence identity to the promoter of another GTP-binding protein, the human c-Ha-ras proto-oncogene. Transcription of the gene was inhibited following treatment of renal cells with 10(-8) M dexamethasone. These studies suggest that alpha-i-2 gene expression is regulated in LLC-PK1 cells. Identification of the gene's promoter and 5'-flanking sequences provide a basis for elucidating "cis-acting" DNA sequences and "trans-acting" protein factors that act in concert to control the transcriptional regulation of the alpha-i-2 gene which may modulate G-protein coupled effector responses in vasopressin-sensitive renal epithelia
