Kinetics of Protein Turnover in Growing Cells of Bacillus megaterium

Abstract

The course of protein degradation during growth of a [14C]leucine-labelled population of BaciZZus megaterium with a surplus of the non-radioactive amino acid indicated the presence of a labile protein fraction decaying with a half-life of less than 1 h. The half-life of the remaining 'stable' fraction was much longer (40 h or more). A nutrient shift-down increased, and a shift-up decreased the relative size of the labile fraction and the rate of degradation of the 'stable' fraction. When bacteria were prelabelled in the presence of ethionine, both the size of the labile fraction and the rate of degradation of the 'stable' fraction were increased. A shift-up in temperature caused a large increase in the size of the labile fraction while the rate of degradation of 'stable' proteins increased only slightly. The rate of degradation of the labile fraction was not changed significantly by any treatment. The results suggest that the main target of regulation of protein turnover by environmental conditions is the relative size of the labile protein fraction. I N T R O D U C T I O NPartial or complete degradation of protein molecules in growing bacterial populations appears to play a role primarily in protein processing or the removal of abnormal proteins, such as the products of translational errors. The half-lives of such processes are probably no longer than several minutes (Pine, 1970; Goldberg, 1972; Bukhari & Zipser, 1973). In a special case, the germination of spores of bacilli, the labile protein fraction is formed by several kinds of proteins, synthesized during sporogenesis; their degradation supplies the germinated spore with amino acids making possible a rapid onset of protein synthesis (Setlow, 1975). However, most proteins in growing bacteria are relatively stable and their degradation rates do not usually exceed 1 % h-' (Goldberg & St John, 1976).The general rate of protein turnover in Escherichia coli is regulated by nutrition as well as by temperature (Pine, 1973). These conclusions were derived mostly from experiments in which the turnover rate was calculated from samples taken at one or two time intervals. Our study of protein turnover in Bacillus megaterium is based on the measurement of the kinetics of protein degradation, from which the proportions of the labile and 'stable' fractions and their degradation constants could be separately determined. We found that the size of the labile fraction, with a half-life of less than 1 h, and to a lesser extent the degradation rate of the 'stable' fraction, are influenced by cultivation conditions. M E T H O D SThe asporogenous strain of Bacillus megaterium KM, and the composition of mineral C medium were described previously (Chaloupka et al., 1975). The medium was supplemented with glucose ( 5 mg ml-I). When the influence of different media was studied, either NH,Cl was omitted and replaced with m-leucine (5 mM) as the sole nitrogen source (C -N + L), or casein hydrolysate (Oxoid; 10mgml-I) was added (C + AA). The media were supplemented with m-leucine (+L; ...

Increased temperature suppresses the translation of mRNA coding for an exocellular proteinase in Bacillus megaterium

“…The relative growth rates at 28°C (taken as 1.0): 35°C : 42°C were 1.0: : 1.6 : 1.7; the relative rates of incorporation of []4C]leucine into proteins were 1.0 : 2.3 : 3.2. The discrepancy between the relative rates of growth and protein synthesis is probably due to the increased rate of protein turnover at higher temperatures [10]. Increased temperature depresses the relative rates of synthesis of exocellular proteinase (Table 1).…”

Protein catabolism in growingBacillus megateriumduring adaptation to salt stress

“…While an increase in protein turnover caused by a severe heat shock (e.g. temperature shift from 28°C to the supraoptimal 42°C) probably reflects largely thermal destabilization of proteins (increase in the proportion of short‐lived cell proteins) [19], salt stress does not influence the protein stability in vivo so strongly, although both these stimuli are fully comparable in the extent of stress protein induction [8]. The initial increase in the degradation of intracellular proteins caused by 1 M NaCl may reflect a physiological adaptation rather than protein destabilization.…”

ATP-dependent and -independent Protein Degradation in Extracts of the Psychrotrophic Bacterium Authrobacter sp. S1 55

“…Therefore, it appears likely that endogenous proteins of Arthrobacter S, 55 were specifically protected in uiuo and in vitro against attack by intracellular proteases. This would correlate with the general observation that proteins with abnormal structures, including shortened chain length, are rapidly and specifically degraded in Arthrobacter S155 (Potier et al, 1985), and a wide range of exponentially growing cells (Carrier et af., 1984;Chaloupka & Strnadova, 1982;Goldberg & St John, 1976;Hipkiss et a f . , 1980;Kemshead & Hipkiss, 1974;Pine, 1972), while the bulk of endogenous proteins remain stable.…”

“…Under optimum growth conditions, the overall rates of protein breakdown within these cells are low (Calandruccio & Larrabee, 1981;Chaloupka & Strnadova, 1982;Willets, 1967), and most of the proteins synthesized during exponential growth are stable (Goldberg & St John, 1976;Larrabee et al, 1980;Pine, 1972). However, changes in growth conditions can cause a rapid and selective degradation of certain enzymes (Rivett et a/., 1985).…”