Molecular properties of a matrix attachment region-binding protein located in the nucleoli of tobacco cells.

We cloned a cDNA for matrix-attachment region (MAR)-binding protein from Nicotiana tabacum cells to elucidate the structure and function of the nuclear matrix. The cDNA encodes a protein of 555 amino acids (61,050 Da) with an isoelectric point of 9.4. We named the protein NtMARBP61. The sequence is 45% identical to yeast Nop58p, which is involved in rRNA processing. The C-terminal part is unique and rich in lysine residues. The recombinant C-terminal part had the ability to bind double-stranded DNAs of 12 tobacco MARs. The intracellular localization was determined to be in the nucleolus by fluorescent microscopy using the antibody to the recombinant NtMARBP61. The mRNA level was high in the lag and early-log phases of cultured cells but low in the stationary phase. The protein was accumulated only in the middle- and late-log phases, suggesting that NtMARBP61 is essential for growing cells. The results suggest at least the structural and regulatory function of NtMARBP61 in the nucleolus as a MAR-binding protein in a growth-stage specific manner.