The Newly Identified Human Nuclear Protein NXP-2 Possesses Three Distinct Domains, the Nuclear Matrix-binding, RNA-binding, and Coiled-coil Domains

Using a monoclonal antibody that recognizes a nuclear matrix protein, we selected a cDNA clone from a λgt11 human placenta cDNA library. This cDNA encoded a 939-amino acid protein designated nuclear matrix protein NXP-2. Northern blot analysis indicated that NXP-2 was expressed in various tissues at different levels. Forcibly expressed green fluorescent protein-tagged NXP-2 as well as endogenous NXP-2 was localized in the nucleus and distributed to the nuclear matrix. NXP-2 was released from the nuclear matrix when RNase A was included in the buffer for nuclear matrix preparation. Mapping of functional domains was carried out using green fluorescent protein-tagged truncated mutants of NXP-2. The region of amino acids 326–353 was responsible for nuclear matrix binding and contained a cluster of hydrophobic amino acids that was similar to the nuclear matrix targeting signal of acute myeloleukemia protein. The central region (amino acids 500–591) was demonstrated to be required for RNA binding by Northwestern analysis, although NXP-2 lacked a known RNA binding motif. The region of amino acid residues 682–876 was predicted to have a coiled-coil structure. The RNA-binding, nuclear matrix-binding, and coiled-coil domains are structurally separated, suggesting that NXP-2 plays important roles in diverse nuclear functions, including RNA metabolism and maintenance of nuclear architecture.