Ubiquitin metabolism in HeLa cells starved of amino acids.

Radio-iodinated ubiquitin (Ub) was introduced into HeLa cells by red blood cell-mediated microinjection. The half-life and solubility of Ub, as well as the molecular weight distributions of Ub conjugates, were then measured in HeLa cells grown in complete medium or in medium lacking amino acids and fetal calf serum. Ub metabolism was similar in the two sets of cells. Thus, the dramatic changes in Ub metabolism induced by thermal stress are not observed upon amino acid deprivation.     

Unbalanced ribosome assembly in Saccharomyces cerevisiae expressing mutant 5 S rRNAs.

Mutant 5 S rRNA genes were expressed in Saccharomyces cerevisiae to further define the function of the ribosomal 5 S RNA. RNA synthesis and utilization were assayed using previously constructed markers which have been shown to be functionally neutral and easily detected by gel electrophoresis. Most mutations were found not to affect the growth rate because they were poorly expressed or could be accommodated effectively in the ribosomal structure. Two of the mutants, Y5A99U56U57 and Y5U90i5 adversely affected cell growth as well as protein synthesis in vitro. Polyribosome profiles in both of these mutants were substantially shorter, and an analysis of the ribosomal subunit composition revealed a significant imbalance with a 25-35% excess in 40 S subunits. Kinetic analyses of RNA labeling indicated very low cellular levels of mutant RNA either because it was poorly expressed (Y5U90i5) or rapidly degraded before being incorporated into mature 60 subunits (Y5A99U56U57). The results suggest that the 5 S RNA is required for the assembly of stable ribosomal 60 S subunits and raise the possibility that this RNA or, more likely, its corresponding ribonucleoprotein complex is critical for subunit assembly or even RNA processing.     

Pigeon metallothionein consists of two species

Two isospecies of metallothionein, a cysteine-rich protein that binds metals, exist in all mammals examined, but only one in some invertebrates and lower animals. Lower vertebrates such as fish and birds have one or two metallothionein genes depending upon the organism. In this study, we show by amino acid sequence determinations that two isospecies of metallothionein, 75% homologous to each other, can be induced by zinc to accumulate in pigeon livers. This is in contrast to single isospecies found in chicken and duck. Each of these two sequences consists of 63 amino acids, with all 20 cysteines in positions held invariant in most if not all class I mammalian metallothioneins. One of these two pigeon isometallothioneins is terminated with histidine at the carboxyl end, which is apparently unique to avians. Its sequence differs from that of duck and chicken by only four substitutions and is the predominant isospecies that accumulates upon induction. The other pigeon metallothionein has lysine at its carboxyl terminus and is devoid of arginine. None of these isospecies carries any aromatic amino acid, which is also characteristic of all higher metallothioneins. As this is the first demonstration with sequence data that two isospecies of metallothionein indeed exist in birds, these results suggest that pigeon metallothionein genes evolved from an ancestral form through duplication and mutation upon specification.     

Studies on the regulatory domain of Ca2+/calmodulin-dependent protein kinase II. Functional analyses of arginine 283 using synthetic inhibitory peptides and site-directed mutagenesis of the alpha subunit

The regulatory role of Arg283 in the autoinhibitory domain of Ca2+/calmodulin-dependent protein kinase II was investigated using substituted inhibitory synthetic peptides and site-directed mutation of the expressed kinase. In the synthetic peptide corresponding to the autoinhibitory domain (residues 281-309) of Ca2+/calmodulin-dependent protein kinase II, substitution of Arg283 by other residues increased the IC50 values of the peptides in the following order: Arg much less than Lys much less than Gln much less than Glu. Site-directed mutations of Arg283 to glutamic acid and glutamine in the kinase alpha subunit cDNA were transcribed and translated in vitro. The expressed enzymes had the same total kinase activities, determined in the presence of Ca2+/CaM, but the Glu283 mutant had a slightly higher Ca2(+)-independent kinase activity (5.46 +/- 0.88%) compared to the wild-type Arg283 (1.86 +/- 0.71%) and the Gln283 mutant (2.15 +/- 0.60%). When the expressed kinases were subjected to limited autophosphorylation on ice to monitor generation of the Ca2(+)-independent activity, the Arg283 kinase attained maximal Ca2(+)-independent activity (about 20%) within 30 s, whereas the Gln283 and Glu283 mutants attained maximal Ca2(+)-independence only after about 40 min of autophosphorylation. The results indicate that Arg283 is a very important determinant for the regulatory autophosphorylation of Thr286 that generates the Ca2(+)-independent activity but is not essential for the other multiple autophosphorylations within Ca2+/calmodulin-dependent protein kinase II, and that Arg283 is only one of several important residues for the inhibitory potency of the autoinhibitory domain.     

Cyclins,cyclin-dependent kinases and differentiation

Cyclin-dependent kinases and their regulatory subunits, the cyclins, are known to regulate progression through the cell cycle. Yet these same proteins are often expressed in non-cycling, differentiated cells. This review surveys the available information about cyclins and cyclin-dependent kinases in differentiated cells and explores the possibility that these proteins may have important functions that are independent of cell cycle regulation.