Localization and functional analysis of HmgB3p, a novel protein containing high-mobility-group-box domain from Tetrahymena thermophila

The high-mobility-group (HMG)-box domain represents a very versatile protein domain that mediates the DNA-binding of non-sequence-specific and sequence-specific proteins. HMG-box proteins are involved in various nuclear functions, including modulating chromatin structure and genomic stability. In this study, we identified the gene HMGB3 in Tetrahymena thermophila. The predicted HmgB3p contained a single HMG-box, an SK-rich-repeat domain and a neutral phosphorylated C-terminal. HMGB3 was expressed in the growth and starvation stages. Furthermore, HMGB3 showed a higher expression levels during the conjugation stage. HMGB3 knockout strains showed no obvious cytological defects, although initiation of HMGB3 knockout strain mating was delayed and maximum mating was decreased. HA-HmgB3p localized on the micronucleus (MIC) during the vegetative growth and starvation stages. Furthermore, HA-HmgB3p specially decorated the meiotic and mitotic functional MIC during the conjugation stage. Truncated HMGB3 lacking the HMG box domain disappeared from MICs and diffused in the cytoplasm. Overexpressed HmgB3p was abnormally maintained in newly developing macronuclei and affected the viability of progeny. Taken together, these results show that HmgB3p is a germline micronuclear-specific marker protein. It may bind to micronucleus-specific DNA sequences or structures and is likely to have some function specific to micronuclei of T. thermophila.