The mouse genome contains two expressed intronless retroposed pseudogenes for the sentrin/sumo-1/PIC1 conjugating enzyme Ubc9

Detection of ubiquityl-calmodulin conjugates with a novel high-molecular weight ubiquitylprotein-isopeptidase in rabbit tissues

The selective degradation of many proteins in eukaryotic cells is carried out by the ubiquitin system. In this pathway, proteins are targeted for degradation by covalent ligation to ubiquitin, a highly conserved protein [1]. Ubiquitylated proteins were degraded by the 26S proteasome in an ATP-depended manner. The degradation of ubiquitylated proteins were controlled by isopeptidase cleavage. A well characterised system of ubiquitylation and deubiquitylation is the calmodulin system in vitro [2]. Detection of ubiquityl-calmodulin conjugtates in vivo have not been shown so far. In this article we discuss the detection of ubiquitin calmodulin conjugates in vivo by incubation with a novel high-molecular weight ubiquitylprotein-isopeptidase in rabbit tissues. Proteins with a molecular weight of ubiquityl-calmodulin conjugates could be detected in all organs tested. Incubation with ubiquitylprotein-isopeptidase showed clearly a decrease of ubiquitin calmodulin conjugates in vivo with an origination of unbounded ubiquitin. These results suggest that only few ubiquitin calmodulin conjugates exist in rabbit tissues.

SUMO--nonclassical ubiquitin

SUMO (small ubiquitin-related modifier) is the best-characterized member of a growing family of ubiquitin-related proteins. It resembles ubiquitin in its structure, its ability to be ligated to other proteins, as well as in the mechanism of ligation. However, in contrast to ubiquitination-often the first step on a one-way road to protein degradation-SUMOlation does not seem to mark proteins for degradation. In fact, SUMO may even function as an antagonist of ubiquitin in the degradation of selected proteins. While most SUMO targets are still at large, available data provide compelling evidence for a role of SUMO in the regulation of protein-protein interactions and/or subcellular localization.

Genomic organization of a human 5beta-reductase and its pseudogene and substrate selectivity of the expressed enzyme

The enzyme 5beta-reductase catalyzes the reduction of the 4-ene of 3-ketosteroids, converting them into 5beta-dihydro-3-ketosteroids and, thus, could be involved in the metabolism of 4-cholestene-3-one, progesterone, 17~-hydroxyprogesterone, aldosterone, corticosterone, cortisol, 4-androstenedione, and testosterone. In this study, we report the genomic structure of a human 5beta-reductase gene, its tissue distribution, the characterization of an intronless pseudogene and the substrate selectivity of the enzyme. The gene coding for the active 5beta-reductase contains nine exons like most members of the aldo-keto reductase family, but the sequence covered by the gene, more than 42 kb, is much longer than the sequence of other members of this family. There are many large introns, especially introns 3, 4 and 7 that span approx. 7 kb, and intron 1 that contains more than 10 kb. Northern blot analysis showed three band sizes of 1.3, 2.2 and 2.7 kb. The 1.3 and 2.7 kb bands are highly expressed in the liver while weaker 2.2 and 1.3 kb bands have been observed in the testis and colon, respectively. We also identified an intronless gene having 86% homology with the 5beta-reductase cDNA sequence. Since its sequence contains many stop codons, this gene is most probably a pseudogene. To determine more precisely the substrate selectivity of the enzyme, we established a stable cell line expressing human 5beta-reductase in transformed embryonic kidney (HEK-293) cells. The transfected cells efficiently catalyze the transformation of progesterone, androstenedione, 17alpha-hydroxyprogesterone and testosterone. However, they catalyze much less efficiently the transformation of compounds containing an 11beta-hydroxy group, such as aldosterone, corticosterone and cortisol. In addition to its role in cholesterol catabolism, it is well recognized that 5beta-reductase inactivates active androgens. Indeed, 5beta-dihydrotestosterone (5beta-DHT), the product of the reduction of testosterone by 5beta-reductase, is not active while its 5~-isomer (DHT) is the most potent natural androgen. Recent findings show that 5beta-pregnanes are active ligands in the induction of CYP3A through the orphan receptor hPAR. Our results thus open an opportunity for studying the new role of 5beta-reductase in the formation of a new type of active steroids.

Ubiquitin-like proteins: new wines in new bottles

Ubiquitin is a small polypeptide that covalently modifies other cellular proteins and targets them to the proteasome for degradation. In recent years, ubiquitin-dependent proteolysis has been demonstrated to play a critical role in the regulation of many cellular processes, such as cell cycle progression, cell signaling, and immune recognition. The recent discovery of three new ubiquitin-like proteins, NEDD8, Sentrin/SUMO, and Apg12, has further broadened the horizon of this type of post-translational protein modification. This review will focus on the biology and biochemistry of the Sentrin/SUMO and NEDD8 modification pathways, which are clearly distinct from the ubiquitination pathway and have unique biological functions.

Functional heterogeneity of small ubiquitin-related protein modifiers SUMO-1 versus SUMO-2/3

Post-translational modification marked by the covalent attachment of the ubiquitin-like protein SUMO-1/SMT3C has been implicated in a wide variety of cellular processes. Recently, two cDNAs encoding proteins related to SUMO-1 have been identified in human and mouse. The functions and regulation of these proteins, known as SUMO-2/SMT3A and SUMO-3/SMT3B, remain largely uncharacterized. We describe herein quantitative and qualitative distinctions between SUMO-1 and SUMO-2/3 in vertebrate cells. Much of this was accomplished through the application of an antibody that recognizes SUMO-2 and -3, but not SUMO-1. This antibody detected multiple SUMO-2/3-modified proteins and revealed that, together, SUMO-2 and -3 constitute a greater percentage of total cellular protein modification than does SUMO-1. Intriguingly, we found that there was a large pool of free, non-conjugated SUMO-2/3 and that the conjugation of SUMO-2/3 to high molecular mass proteins was induced when the cells were subjected to protein-damaging stimuli such as acute temperature fluctuation. In addition, we demonstrated that SUMO-2/3 conjugated poorly, if at all, to a major SUMO-1 substrate, the Ran GTPase-activating protein RanGAP1. Together, these results support the concept of important distinctions between the SUMO-2/3 and SUMO-1 conjugation pathways and suggest a role for SUMO-2/3 in the cellular responses to environmental stress.

SUMO/sentrin: protein modifiers regulating important cellular functions

Regulation of protein functions can be achieved by posttranslational protein modifications. One of the most studied modifications has been conjugation to ubiquitin, which mainly targets substrate proteins for degradation by the 26 S proteasome. Recently, SUMO/sentrin, a ubiquitin-like protein has been characterized. This evolutionary conserved protein is conjugated to specific proteins in a way similar, but not identical, to ubiquitin and seems also to be involved in the regulation of protein localization or function. An increasing number of SUMO/sentrin substrates are currently described. We focus here on three major substrates of modification by SUMO: RanGAP1, PML, and IkappaBalpha proteins. These different examples illustrate how SUMO conjugation may be involved in the control of the level of critical proteins within the cell or in the modulation of subcellular localization and nucleocytoplasmic trafficking.

RNAs from all categories generate retrosequences that may be exapted as novel genes or regulatory elements

While the significance of middle repetitive elements had been neglected for a long time, there are again tendencies to ascribe most members of a given middle repetitive sequence family a functional role--as if the discussion of SINE (short interspersed repetitive elements) function only can occupy extreme positions. In this article, I argue that differences between the various classes of retrosequences concern mainly their copy numbers. Consequently, the function of SINEs should be viewed as pragmatic such as, for example, mRNA-derived retrosequences, without underestimating the impact of retroposition for generation of novel protein coding genes or parts thereof (exon shuffling by retroposition) and in particular of SINEs (and retroelements) in modulating genes and their expression. Rapid genomic change by accumulating retrosequences may even facilitate speciation [McDonald, J.F., 1995. Transposable elements: possible catalysts of organismic evolution. Trends Ecol. Evol. 10, 123-126.] In addition to providing mobile regulatory elements, small RNA-derived retrosequences including SINEs can, in analogy to mRNA-derived retrosequences, also give rise to novel small RNA genes. Perhaps not representative for all SINE/master gene relationships, we gained significant knowledge by studying the small neuronal non-messenger RNAs, namely BC1 RNA in rodents and BC200 RNA in primates. BC1 is the first identified master gene generating a subclass of ID repetitive elements, and BC200 is the only known Alu element (monomeric) that was exapted as a novel small RNA encoding gene.