Nucleolus contains signal molecules that constitute membrane-nucleolus linked pathway

Abstract

Since there have been very few studies on nucleolar signaling, an attempt was made to establish nucleolar signal pathways which link the cell membrane to the nucleolus for the transfer of extracellular signals. Tw o pathways were studied. One was the G s mediated cAMP pathway where two signal molecules were yielded, including R I I and protein kinase A. The other was the G q mediated DAG/IP 3 pathway which yields two signals including protein kinase C and I P 3 / C a 2 + . By the studying isolated nucleoli from resting liver, regenerating liver or weak carcinogen thioacetamide treated liver, it was possible to detect protein kinase A (PKA), protein kinase C (PKC) and R I I subunits. In addition, CK2 was detected. It was found that external signals transmitted through G protein coupled recep-tors could reach into the nucleolus and that physical translocation of signal molecules was an integral step involved in membrane-nucleolus linked pathways. When an i n vitro assay of the above signal molecules w a s carried out using [ -3 2 P ] -AT P, most kinase dependent phosphorylation was via the major CK2 (more than 95%). Therefore, it is suggested that the major CK2 dependent pathway is involved in 'house keeping' for nucleolar integrity and the minor pathways, dependent on PKA, PKC and others, are involved in subtle regulatory mechanisms such as 'extra-house-keeping' activities by nucleolar chromosomal remodeling. IntroductionThe cell nucleus has three transcriptional substructures where pol I dependent transcription is solely performed in the nucleolus, while pol II and pol III dependent transcriptions take place in the nucleoplasm. Being the site of pol I dependent ribosome genesis, the mammalian nucleolus contains a number of post-translational modification mechanisms involved in the activation-deactivation cycle related to the regulation of nucleolar ribosome genesis. These mechanisms are protein methylation (Lischwe et al, 1985), acetylation (Goldknopf et al, 1979), ubiquitination (Goldknopf andBusch, 1978), phosphorylation (Olson et al, 1974), adduct formation to RNA and others (Busch, 1997). Among these molecular mechanisms, protein phosphorylation is a major modification process involved in the regulation of nucleolar function. In earlier works using [ 3 2 P ] -l a b e l i n g techniques of tissue culture, three major nucleolar phosphoproteins were defined. These were 38 kDa nucleophosmin, 110 kDa nucleolin and 140 kDa Nopp140 . More recently, sequence studies of various nucleolar proteins made it possible to define a dozen species of nucleolar phosphoproteins (Busch, 1997).R e c e n t l y, an extensive effort has been made to characterize the mechanism of kinase dependent nucleolar protein phosphorylation involved in various biological conditions. Despite the fact that the research area of signal transduction has advanced enormously as a whole, there were very few studies of the nucleolar signaling system compared to the nuclear signaling system (Meck and Street, 1992;Inagaki et al, 1994;Buchner, 1...

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