Mesangial cells play an important role in maintaining glomeruli structure and function and in the pathogenesis of glomerular diseases. With a novel approach using a rapid large-scale DNA sequencing strategy and computerized data processing, a new human gene, PP4(Rmeg) was cloned. The full-length cDNA 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). Recombinant PP4(Rmeg) produced in COS-7 cells bound to the catalytic subunit of PP4. PP4(Rmeg) is therefore structurally and functionally related to the 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 their expression was relatively ubiquitous. In situ hybridization studies in normal human renal tissues confirmed their expression in glomeruli in vivo. The 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 abundantly in mesangial cells. Its pathophysiologic role in mesangial cells and glomerulus remains unknown. As PP4 is an essential protein for nucleation, growth, and stabilization of microtubules at centrosomes/spindle pole bodies during cell division, PP4(Rmeg) may play a role in regulation of mitosis in mesangial cells.