Polyanionic stretch-deleted histone chaperone cia1/Asf1p is functional both in vivo and in vitro

CIA, an interactor of the CCG1 histone acetyltransferase subunit of TFIID, was identified as a human histone chaperone. The Saccharomyces cerevisiae orthologue ASF1, when it was over-expressed, was reported to cause de-repression of silent loci; however, the involvement of Asf1p in the alteration of nucleosomal structures remained unknown. Curiously, there is a polyanionic stretch, a structural motif characteristic of histone chaperones, in S. cerevisiae Asf1p, but not in human CIA. We investigated how CIA/Asf1p utilizes its domain(s) for the alteration of nucleosomal structure.

To characterize the relationships between the domain structures and nuclear functions of CIA, we isolated the gene for the CIA counterpart in Schizosaccharomyces pombe, designated cia1+, whose putative product contains a polyanionic stretch. Gene disruption of cia1+ was lethal, which is the distinct phenotype of viable S. cerevisiae asf1. The cia1- lethality was rescued by the introduction of S. cerevisiae ASF1, but not by the introduction of human CIA cDNA. To our surprise, the construct that produces Asf1p, lacking the polyanionic stretch, is capable of rescuing the lethality caused by the cia1+ deletion, while the highly conserved N-terminal region of Asf1p is essential for the complementation of cia1- growth defects. The polyanionic stretch-deleted Asf1p is sufficient both for interaction with histones H3/H4 and for nucleosome assembly in vitro, as well as for telomeric de-repression in vivo.

These findings suggest that the areas responsible for both the conserved and species-specific functions of CIA/cia1/Asf1p are within their highly conserved regions and that the yeast-specific polyanionic stretch of cia1/Asf1p is not necessary for viability, histone binding, nucleosome assembly, or anti-silencing.