Ubiquitin gene expression and ubiquitin conjugation in chicken muscle do not reflect differences in growth rate between broiler and layer birds

Previous work has shown that chicken strains selected for growth (broilers) degrade muscle proteins less rapidly than those selected for egg laying. Th ey also have decreased calpain and increased calpastatin content iu breast muscle. This study aimed to test the hypothesis that these differences corr elate with changes in the ATP- and ubiquitin-dependent proteolytic system. Chickens of a broiler strain (Ross 1) and a layer strain (ISABrown) were re ared to the age of 4 wk under identical conditions with ad libitum access t o feed and water. Mean fractional growth rates were 10.4%/d for broilers an d 7.4%/d for layers. Feed intake measured in the last week of the trial was slightly greater in layer birds (.11 and .12 g.g body weight(-1).d(-1) for broilers and layers respectively; P < .006); Polyubiquitin (UbI) messenger RNA was abundant in the muscles of these well-fed birds, but it showed lit tle difference between strains. Muscle did not significantly express the Ub II polyubiquitin gene. The ATP-dependent system conjugating ubiquitin to en dogenous proteins had greatest activity in the gastrocnemius muscle of broi ler birds but was not significantly different between breeds. Proteins cros s-reactive with antisera to recombinant human proteasome regulatory subunit s MSS1 (multicopy suppressor of SUG 1; S7) and TBP1 (tat binding protein 1; S6') were present in muscle homogenates from both strains of bird. The chi ck equivalent of TBP1 was more abundant in breast muscle of broiler birds t han in leg muscle, or in either muscle of layers. Antiserum to recombinant yeast subunit mts2 (mitosis temperature sensitive gene 2; S4) did not react with any protein of the expected size but detected a 30-kDa peptide that w as not associated with the 26S proteasome; this was found only in muscle fr om the layer strain. Hence, during normal growth of chickens, rates of prot ein degradation are not controlled by the expression of ubiquitin mRNA or t he conjugation of ubiquitin. However, the composition of the 26S proteasome may be a regulatory factor.