With the recent molecular findings, the podocyte is emerging as a key cell type involved in glomerular damage, but protein complexes involved remain poorly understood. To systematically search for additional podocyte molecules interacting with nephrin, a key structural molecule of the interpodocyte filtration slit, precipitation of glomerular lysates was set out with anti-nephrin antibodies to identify members of the nephrin-associated protein complex. Proteins of the precipitate were subsequently identified with MALDI-TOF mass analysis. One of the proteins thus obtained showed identity with densin, a protein originally purified from rat forebrain postsynaptic density fraction and so far shown to be highly brain-specific. The expression of densin appeared distinctly in the glomerulus and cultured podocytes by RT-PCR. Immunoblotting studies revealed a specific band of 185 kD in brain and cultured podocytes; in human glomerulus, densin appeared as a 210-kD band. By immunocytochemistry, densin localizes in glomeruli in a podocyte-like pattern. Electron microscopic studies revealed densin localization in the slit diaphragm area. Due to its known involvement in the synaptic organization, maintenance of cell shape and polarity in nerve cells, together with its demonstrated interactions with alpha-actinin-4, densin may share the same functions in podocytes by associating with the nephrin interacting protein complex at the slit diaphragm.