We have cloned and characterized a human cDNA, designated N4WBP5A, that belongs to the family of Nedd4-binding proteins. We originally identified N4WBP5A as an unknown expressed sequence tag (AA770150) represented in a cDNA microarray analysis that was up-regulated upon activation of T cells and inhibited by cell treatment with the calcineurin phosphatase inhibitors, cyclosporine (CsA) and tacrolimus (FK506). The predicted N4WBP5A amino acid sequence of 242 amino acid residues reveals an open reading frame of 729 nucleotides with a corresponding molecular mass of 27.1 kDa. Detection of N4WBP5A mRNA by reverse transcription-PCR was consistent with the induction of N4WBP5A following mitogenic stimulation of T lymphocytes and inhibition by CsA. Immunoblot analysis revealed endogenous N4WBP5A protein to be up-regulated following T cell activation and inhibited by CsA. This regulation of N4WBP5A mRNA expression differed from that of its homologue (51% identical; 65% similar) N4WBP5. Like N4WBP5, however, expression of epitope-tagged N4WBP5A indicated that the protein is localized predominantly to the Golgi network. Here we show by co-precipitation experiments that N4WBP5A interacts with the WW domains of Nedd4, an E3 ubiquitin ligase. Taken together, our data suggest that N4WBP5A may play a regulatory role in modulating Nedd4 activity at the level of the Golgi apparatus in T lymphocytes.