Identification and characterization of a SET/NAP protein encoded by a brain-specific gene, MB20

A new member of the NAP/SET gene family, named MB20, was isolated from a mo use brain cDNA library by virtue of its GAG: trinucleotide repetitive seque nce and a brain-specific gene expression pattern. The complementary DNA seq uence predicted an open reading frame of 545 amino acids, with four copies of an 11-amino-acid direct repeat. The consensus sequence for these repeats , PKE-P--K-EE, is present in the largest subunit of murine neurofilament (N F-H). The MB20 protein sequence is homologous to nucleosome assembly protei ns of several species, and its C-terminus is homologous to SET proteins. Im munoblot analysis revealed that MB20 protein is expressed in the brain. Tra nsient transfection and immunofluorescence microscopy demonstrated that MB2 0 is distributed in the cytoplasm as well as in the nucleus. Deletion of th e N-terminal end imparts the complete localization of MB20 protein to the n ucleus. The ability of MB20 to bind histone proteins was analyzed by sucros e gradient sedimentation and by retention of histone proteins by immobilize d MB20 protein. On the basis of its expression pattern, predicted sequence, and protein properties, we propose that MB20 plays a unique role in modula ting nucleosome structure and gene expression during brain development. (C) 2001 Academic Press.