Ubiquitin moves to the cell surface

Ubiquitin genes as a paradigm of concerted evolution of tandem repeats

Ubiquitin is remarkable for its ubiquitous distribution and its extreme protein sequence conservation. Ubiquitin genes comprise direct repeats of the ubiquitin coding unit with no spacers. The nucleotide sequences of several ubiquitin repeats from each of humans, chicken, Xenopus, Drosophila, barley, and yeast have recently been determined. By analysis of these data we show that ubiquitin is evolving more slowly than any other known protein, and that this (together with its gene organization) contributes to an ideal situation for the occurrence of concerted evolution of tandem repeats. By contrast, there is little evidence of between-cluster concerted evolution. We deduce that in ubiquitin genes, concerted evolution involves both unequal crossover and gene conversion, and that the average time since two repeated units within the polyubiquitin locus most recently shared a common ancestor is approximately 38 million years (Myr) in mammals, but perhaps only 11 Myr in Drosophila. The extreme conservatism of ubiquitin evolution also allows the inference that certain synonymous serine codons differing at the first two positions were probably mutated at single steps.

The human ubiquitin gene family: structure of a gene and pseudogenes from the Ub B subfamily

An ubiquitin cDNA clone was isolated from a human liver cDNA library. This clone contained two complete, and a portion of a third, ubiquitin coding sequences joined head to tail with no spacer peptides. Screening a human genomic library with a probe derived from the coding region of this cDNA identified a large number of cross-hybridising clones. Differential screening of these genomic clones with the 3' non-coding region of the cDNA identified three different 3'-positive clones. Sequence analysis of these three clones revealed: a gene corresponding to the cDNA containing an intron in the 5' non-coding region and coding for three direct repeats of mature ubiquitin, and two related pseudogenes which appear to have arisen by reverse transcription and insertion into the genome. However, one pseudogene contains two repeats of the ubiquitin coding sequence, while the other contains only one. Hybridisation analysis of restricted human genomic DNA suggests the presence of one other closely related gene within the genome.

Catabolism of intracellular protein: molecular aspects

All living cells regulate the content and composition of their resident proteins, but the mechanisms by which this is accomplished are not understood. The process of protein degradation has an important role in determining steady state and fluctuations of protein concentrations in mammalian cells. This process may be regulated by innate properties of the protein substrates, by factors that interact or "brand" proteins for degradation or by the degradative machinery of the cell. For a specific protein, there appears to be a committed step, an irreversible event that leads to rapid and extensive degradation. That initial event may or may not involve 1) proteolysis, 2) a nonproteolytic covalent modification or branding event (e.g., oxidation, ubiquitin conjugation), 3) denaturation or unfolding of the protein, or 4) sequestration. The degradative machinery of cells may either recognize proteins committed to degradation or initiate degradation, but the process must be selective because there is great heterogeneity in the rates of degradation for different proteins of one cell. The degradative process certainly requires proteases, and it is probable that lysosomal and extralysosomal proteases are involved in the catabolism of cellular proteins. We review here briefly what is currently known about the factors that may determine the half-life of a protein in a mammalian cell, the role of the protein substrate and sequestration in the process, the proteolytic and nonproteolytic enzymes that may initiate the degradative process, and the regulation of extensive degradation of proteins in cells.