Involvement of Nucleocytoplasmic Shuttling of Yeast Nap1 in Mitotic Progression

Abstract

Nucleosome assembly protein 1 (Nap1) is widely conserved from yeasts to humans and facilitates nucleosome formation in vitro as a histone chaperone. Nap1 is generally localized in the cytoplasm, except that subcellular localization of Drosophila melanogaster Nap1 is dynamically regulated between the cytoplasm and nucleus during early development. The cytoplasmic localization of Nap1 is seemingly incompatible with the proposed role of Nap1 in nucleosome formation, which should occur in the nucleus. Here, we have examined the roles of a putative nuclear export signal (NES) sequence in yeast Nap1 (yNap1). yNap1 mutants lacking the NES-like sequence were localized predominantly in the nucleus. Deletion of NAP1 in cells harboring a single mitotic cyclin gene is known to cause mitotic delay and temperature-sensitive growth. A wild-type NAP1 complemented these phenotypes while nap1 mutant genes lacking the NES-like sequence or carboxy-terminal region did not. These and other results suggest that yNap1 is a nucleocytoplasmic shuttling protein and that its shuttling is important for yNap1 function during mitotic progression. This study also provides a possible explanation for Nap1's involvement in nucleosome assembly and/or remodeling in the nucleus.

ACKNOWLEDGMENTS

We thank Doug Kellogg for providing yeast strains and helpful discussion. kap114Δ strains were kindly provided by Steve Buratowski and Pamela A. Silver. We thank Yoshimasa Kiyomatsu (Olympus ProMarketing, Inc.) for permitting us to use the laser scanning cytometer and for technical support. We thank Yuki Yamaguchi for critical comments on the manuscript.

This work was supported by a grant-in-aid from the Ministry of Education, Culture, Sports, Science, and Technology of Japan and by a grant from the Bioarchitect Research Project of RIKEN.