Cloning and characterization of a novel subunit of protein serine/threonine phosphatase 4 from mesangial cells

Mesangial cells play an important role in maintaining glomeruli structure a nd function and in the pathogenesis of glomerular diseases. With a novel ap proach using a rapid large-scale DNA sequencing strategy and computerized d ata processing, a new human gene, PP4(Rmeg) was cloned. The full-length cDN A clone of human PP4(Rmeg) coded for a novel 950-amino acid protein, which was similar to a subunit of protein serine/threonine phosphatase 4 (PP4). R ecombinant PP4(Rmeg) produced in COS-7 cells bound to the catalytic subunit of PP4. PP4(Rmeg) is therefore structurally and functionally related to th e recently reported regulatory subunit of PP4, PP4(R1). Amino acid sequence analysis of rat PP4(Rmeg) homologue revealed that the sequences were well conserved between human and rat (86.3% identity). Northern blot analyses of human tissues and cultured cells demonstrated that the regulatory subunits were expressed abundantly in human cultured mesangial cells, although thei r expression was relatively ubiquitous. In situ hybridization studies in no rmal human renal tissues confirmed their expression in glomeruli in vivo. T he expression was upregulated in glomeruli of anti-Thy1 glomerulonephritis rats before mesangial proliferation. These data demonstrate that PP4(Rmeg) is a novel regulatory subunit of PP4, which is expressed ubiquitously but a bundantly in mesangial cells. Its pathophysiologic role in mesangial cells and glomerulus remains unknown. As PP4 is an essential protein for nucleati on, growth, and stabilization of microtubules at centrosomes/spindle pole b odies during cell division, PP4(Rmeg) may play a role in regulation of mito sis in mesangial cells.