The human EBNA-2 coactivator p100: multidomain organization and relationship to the staphylococcal nuclease fold and to the tudor protein involved in Drosophila melanogaster development.

The human p100 protein was recently identified as a coactivator of the Epstein-Barr virus nuclear antigen 2. On the basis of sequence analysis with the hydrophobic cluster analysis method, we predict that this protein consists of a repeat of four similar domains. Their fold can be related to the staphylococcal nuclease structure whose first subdomain belongs to the large oligonucleotide/oligosaccharide-binding (OB)-fold superfamily. The catalytic amino acids present in nucleases are missing, however, suggesting that these repeated motifs could only serve to bind DNA without catalytic activity, as in many other OB-folds. A highly modified fifth domain follows the four nuclease-like domains, conserving the second subdomain of the nuclease structure but not the first one (the OB-fold), which is replaced by an original domain found in multiple copies in the tudor protein, a Drosophila melanogaster protein required during oogenesis for establishment of a functional posterior organizing centre. We named this heretofore undescribed domain the 'tudor domain' and highlight within it five invariant residues which could be involved in one of the essential roles played by these proteins.