The levels of ubiquitinated histone H2A are highly upregulated in transformed human cells: partial colocalization of uH2A clusters and PCNA/cyclin foci in a fraction of cells in S-phase

Histone ubiquitination and chromatin remodeling in mouse spermatogenesis

Male infertility in HR6B knockout mice is associated with impairment of spermatogenesis. The HR6B gene is a mammalian, autosomal homolog of the Saccharomyces cerevisiae gene Rad6 encoding a ubiquitin-conjugating enzyme. In addition, X-chromosomal HR6A has been identified, in human and mouse. RAD6 in yeast is required for a variety of cellular functions, including sporulation, DNA repair, and mutagenesis. Since RAD6 and its mammalian homologs can ubiquitinate histones in vitro, we have investigated the pattern of histone ubiquitination in mouse testis. By immunoblot and immunohistochemical analysis of wild-type mouse testis, a high amount of ubiquitinated H2A (uH2A) was detected in pachytene spermatocytes. This signal became undetectable in round spermatids, but then increased again during a relatively short developmental period, in elongating spermatids. No other ubiquitinated histones were observed. In the HR6B knockout mice, we failed to detect an overt defect in the overall pattern of histone ubiquitination. For somatic cell types, it has been shown that histone ubiquitination is associated with destabilization of nucleosomes, in relation to active gene transcription. Unexpectedly, the most intense uH2A signal in pachytene spermatocytes was detected in the sex body, an inactive nuclear structure that contains the heterochromatic X and Y chromosomes. The postmeiotic uH2A immunoexpression in elongating spermatids indicates that nucleosome destabilization induced by histone ubiquitination may play a facilitating role during histone-to-protamine replacement.

The measurement of ubiquitin and ubiquitinated proteins

Ubiquitination of key cellular proteins involved in signal transduction, gene transcription and cell-cycle regulation usually condemns those proteins to proteasomal or lysosomal degradation. Additionally, cycles of reversible ubiquitination regulate the function of certain proteins in a manner analogous to phosphorylation. In this short review we describe the current methodology for measuring ubiquitin and ubiquitination, provide examples which illustrate how various techniques have been used to study protein ubiquination, alert the readers of pitfalls to avoid, and offer guidelines to investigators newly interested in this novel post-translational protein modification.

Intracellular proteinases of invertebrates: calcium-dependent and proteasome/ubiquitin-dependent systems

Cytosolic proteinases carry out a variety of regulatory functions by controlling protein levels and/or activities within cells. Calcium-dependent and ubiquitin/proteasome-dependent pathways are common to all eukaryotes. The former pathway consists of a diverse group of Ca(2+)-dependent cysteine proteinases (CDPs; calpains in vertebrate tissues). The latter pathway is highly conserved and consists of ubiquitin, ubiquitin-conjugating enzymes, deubiquitinases, and the proteasome. This review summarizes the biochemical properties and genetics of invertebrate CDPs and proteasomes and their roles in programmed cell death, stress responses (heat shock and anoxia), skeletal muscle atrophy, gametogenesis and fertilization, development and pattern formation, cell-cell recognition, signal transduction and learning, and photoreceptor light adaptation. These pathways carry out bulk protein degradation in the programmed death of the intersegmental and flight muscles of insects and of individuals in a colonial ascidian; molt-induced atrophy of crustacean claw muscle; and responses of brine shrimp, mussels, and insects to environmental stress. Selective proteolysis occurs in response to specific signals, such as in modulating protein kinase A activity in sea hare and fruit fly associated with learning; gametogenesis, differentiation, and development in sponge, echinoderms, nematode, ascidian, and insects; and in light adaptation of photoreceptors in the eyes of squid, insects, and crustaceans. Proteolytic activities and specificities are regulated through proteinase gene expression (CDP isozymes and proteasomal subunits), allosteric regulators, and posttranslational modifications, as well as through specific targeting of protein substrates by a diverse assemblage of ubiquitin-conjugases and deubiquitinases. Thus, the regulation of intracellular proteolysis approaches the complexity and versatility of transcriptional and translational mechanisms.

Peptides bound to major histocompatibility complex molecules

Peptides are the means by which immune effector T cells recognize and defend against the foreign proteins of pathogens. T cell recognition of these molecules, however, is strictly dependent on peptide binding to the receptor-like molecules of the major histocompatibility complex (MHC) locus. The basic unit of recognition is a trimolecular complex consisting of the T cell antigen receptor, the MHC molecule, and the MHC-bound peptide ligand. The multistep process that culminates in MHC presentation of peptides to T cells begins in the last phases of protein catabolism. While the individual roles of many key molecules involved in peptide presentation have recently been defined, there still remain many questions regarding processing of proteins into MHC-bound peptides. This review summarizes the recent developments in peptide antigen processing for MHC molecules, with focus on how proteins are believed to be sampled and selected for degradation into peptides.

MHC class I antigen processing pathways

The multistep process that culminates in major histocompatibility complex (MHC) class I presentation of foreign of self-peptides begins in the last phases of protein catabolism. Although the individual roles of many key molecules-such as proteasomes, the transporter associated with antigen processing, and various endoplasmic reticulum chaperones-have recently been elucidated, there still remain many questions regarding processing of proteins into MHC class I bound peptides. This review summarizes the recent developments in antigen processing for MHC class I molecules, with a focus on how proteins are believed to be sampled and selected for degradation.

SUG1, a putative transcriptional mediator and subunit of the PA700 proteasome regulatory complex, is a DNA helicase

Yeast SUG1 was originally characterized as a transcriptional mediator for the GAL4 transactivator. A similar role in vertebrates was suggested by the ligand-enhanced interaction between mammalian homologues of yeast SUG1 and the ligand-dependent activating domain (AF-2) of nuclear receptors. SUG1 was also shown to be a component of the PA700 regulatory complex of the 26 S proteasome and a member of a large family of putative ATPases. However, no catalytic function has yet been attributed to SUG1. We show here that SUG1 is a 3'-5' DNA helicase whose activity is dependent on an intact ATP binding domain. The sedimentation heterogeneity of mammalian SUG1 suggests that it may be associated with distinct protein complexes and therefore play multiple roles.