BACKGROUND: Inducible inactivation of a protein is a powerful approach for 
analysis of its function within cells. Fission yeast is a useful model for 
studying the fundamental mechanisms such as chromosome maintenance and cell 
cycle. However, previously published strategies for protein-depletion are 
successful only for some proteins in some specific conditions and still do not 
achieve efficient depletion to cause acute phenotypes such as immediate cell 
cycle arrest. The aim of this work was to construct a useful and powerful 
protein-depletion system in Shizosaccaromyces pombe.
RESULTS: We constructed an auxin-inducible degron (AID) system, which utilizes 
auxin-dependent poly-ubiquitination of Aux/IAA proteins by SCFTIR1 in plants, in 
fission yeast. Although expression of a plant F-box protein, TIR1, decreased 
Mcm4-aid, a component of the MCM complex essential for DNA replication tagged 
with Aux/IAA peptide, depletion did not result in an evident growth defect. We 
successfully improved degradation efficiency of Mcm4-aid by fusion of TIR1 with 
fission yeast Skp1, a conserved F-box-interacting component of SCF (improved-AID 
system; i-AID), and the cells showed severe defect in growth. The i-AID system 
induced degradation of Mcm4-aid in the chromatin-bound MCM complex as well as 
those in soluble fractions. The i-AID system in conjunction with transcription 
repression (off-AID system), we achieved more efficient depletion of other 
proteins including Pol1 and Cdc45, causing early S phase arrest.
CONCLUSION: Improvement of the AID system allowed us to construct conditional 
null mutants of S. pombe. We propose that the off-AID system is the powerful 
method for in vivo protein-depletion in fission yeast.