Characterization and Functional Aspects of Human Ninein Isoforms that Regulated by Centrosomal Targeting Signals and Evidence for Docking Sites to Direct Gamma-Tubulin

The functions of centrosomal protein ninein may be involved in microtubule minus end capping, centriole positioning, protein anchoring, and microtubule nucleation, but the true physiological function of various human hNinein isoforms remains to be determined. Here we describe the identification of four diverse CCII-termini of human hNinein isoforms, including a novel isoform 6, by differential expression in a tissue-specific manner. These hNinein isoforms exhibit centrosomal (concentrated) and noncentrosomal (aggregated) localization when GFP-tagged fusion proteins are expressed transiently in mammalian cells. In a kinase assay, we show that the CCII region of hNinein provides a differential phosphorylation site by GSK3β. In addition, our data indicate that either N-terminal or CCIIZ domain disruption may cause hNinein conformational change which recruits γ-tubulin to centrosomal or non-centrosomal hNinein-containing sites, implying that the γ-tubulin localization may be hNinein-dependent. Further, our RNA interference experiment against all hNinein isoforms caused a significant decrease in the γ-tubulin signal in the centrosome. In domain swapping, we clearly show that the CCIIX-CCIIY region provides docking sites for γ-tubulin. Moreover, our data also show that nucleation of microtubules from the centrosome is significantly affected by the presence of either the full-length hNinein or CCIIX-CCIIY region overexpression. Taken together, these results show that the centrosomal targeting signals of hNinein have a role not only in regulating hNinein conformation, resulting in localization change, but also provide docking sites to recruit γ-tubulin at centrosomal and non-centrosomal sites.