Role of nuclear WW domains and proline-rich proteins in dinoflagellate transcription

Dinoflagellates are unique among eukaryotes in their lack of histones and n ucleosomes, and permanently condensed chromosomes. These unusual features r aise questions as how chromatin condensation and gene expression are achiev ed. In this study, we investigated nuclear proteins potentially implicated in the regulation of the transcription. Dinap1 is a dinoflagellate nuclear protein that has a WW domain and is synthesized mainly in G1 and S phases o f the cell cycle. In this study, we found that Dip1, a proline-rich potenti al ligand of Dinap1, and DapC, a Dip1 potential ligand, were both present i n the nucleus of Crypthecodinium cohnii during the G1 phase. Dip1 contained a PPXY motif, and its domain organization was similar to that of the splic ing factor FBP21 in that it possessed one zinc finger and two WW domains. A lthough DapC has no known homolog, 22 repeats of a PPXPXGX heptapeptide wer e identified at the N-terminus, and this structure is similar to that of th e C-terminal part of the mouse splicing factor SAP62. Dinap1 was co-precipi tated with Dip1 and DapC in vitro and in vivo, but despite their nuclear lo cation, these three proteins did not bind directly to DNA. Dinap1 activated up to 40% of the basal transcription activity of C. cohnii in an in vitro assay, whereas DapC inhibited it by 40% and Dip1 had no eff ect. These dinoflagellate proteins appear to be the subunits of a nuclear c omplex that may be involved in regulating transcription.