In addition, an increased level of certain histones, including acetylated histone H4, remain in the elongating spermatids of PA200 deficient mice [49]. following characteristics. It is mediated through the sequential activities of three ubiquitinating enzymes: ubiquitin-activating enzyme (E1), ubiquitin-conjugating enzyme (E2) and ubiquitin ligase (E3) [1]. To date, broad range of proteins have been reported to undergo ubiquitination, including receptors, transcription factors, ion channels, signaling molecules, cytoskeletal components and unnecessary/damaged proteins. E3 enzymes are key factors that determine substrate specificity, and for this reason the human genome encodes more than 600 E3s [2]. In addition to the high number of E3s, the different modes of ubiquitin conjugation reflect the functional diversity of protein ubiquitination. Proteins can be modified by addition of single ubiquitin molecule (monoubiquitination) or a polymer of ubiquitin (polyubiquitination). Ubiquitin contains seven lysine residues (K6, K11, K27, K29, K33, K48 and K63) that can be conjugated to another ubiquitin 3-Methylcrotonyl Glycine molecule, thereby forming at least seven different polyubiquitin linkages [3]. A K48-linked polyubiquitin chain generally serves as a signal for protein degradation by the proteasome. Monoubiquitination and K63-linked ubiquitination have various non-proteasomal functions, such as endocytosis, protein trafficking and DNA repair. Other ubiquitin linkages are comparatively minor, but they appear to function in proteasomal degradation and DNA repair [4]. Ubiquitination is counteracted by deubiquitinating enzymes that remove ubiquitin from protein substrates [5]. The molecular mechanism and physiological significance of ubiquitin-mediated processes have been extensively studied in yeast,Drosophilaand mammals, but have been much less investigated in germ cells. However, over the years, an increasing number of studies have emphasized the importance of the ubiquitin system in male 3-Methylcrotonyl Glycine gametogenesis (spermatogenesis) and fertilization. Sperm have unique membranous organelles specialized for sperm motility and penetration, that is, a condensed nucleus, an acrosome and helically arranged mitochondria. Defects in organization and/or integrity of these organelles are closely associated with impaired sperm function and male infertility. In somatic cells, protein ubiquitination plays a central role in the regulation of the morphology and function of membranous organelles, for example, protein quality control in the endoplasmic reticulum (ER), protein sorting in endosomes and mitochondrial dynamics [6,7,8]. Likewise, it could be important for the maintenance of the integrity of sperm 3-Methylcrotonyl Glycine organelles. This review will focus on recent advances in the state of knowledge concerning the role of protein ubiquitination in biogenesis and the function and stability of sperm membranous organelles. == 2. Ubiquitinating and Deubiquitinating Enzymes during Spermatogenesis == Spermatogenesis is a complex and dynamic process by which the metamorphosis of male germ cells into mature spermatozoa takes place. During mammalian spermatogenesis, spermatogonial stem cells proliferate and differentiate into spermatocytes by mitotic division. Subsequently, diploid spermatocytes undergo meiosis and differentiate into haploid round spermatids. Round spermatids transform into elongated spermatids through a unique differentiation process called spermiogenesis, and then eventually develop into mature spermatozoa. Spermiogenesis includes nuclear shaping, acrosome biogenesis, flagellum formation, mitochondrial rearrangement and cytoplasmic trimming (Figure 1) [9]. Several testis transcriptome studies have shown that the complex process of spermatogenesis is regulated by a highly integrated mechanism that involves changes in gene expression in a developmental stage-dependent Mouse monoclonal antibody to Hexokinase 2. Hexokinases phosphorylate glucose to produce glucose-6-phosphate, the first step in mostglucose metabolism pathways. This gene encodes hexokinase 2, the predominant form found inskeletal muscle. It localizes to the outer membrane of mitochondria. Expression of this gene isinsulin-responsive, and studies in rat suggest that it is involved in the increased rate of glycolysisseen in rapidly growing cancer cells. [provided by RefSeq, Apr 2009] manner [10,11,12]. An increasing number of gene products have been found to have specialized biological functions required for the different phases of spermatogenesis. Indeed, a recent proteomics study identified more than 4,600 human sperm proteins, of which approximately 220 are sperm-specific [13]. Since ubiquitin was first isolated from the testes of trout and mammals [14,15], more than 30 ubiquitinating enzymes have been identified as important regulators of spermatogenesis [16]. It is estimated that approximately 70 E3 ubiquitin ligases are expressed during spermatogenesis in mice [17], suggesting that the ubiquitin system has diverse functions. Further investigation is needed to address the physiological roles of the as yet uncharacterized sperm E3s. Mammals contain ~90 deubiquitinating enzymes, many of which are associated with pathogenesis [18]. At present, little is known about the expression and physiological roles of deubiquitinating enzymes in male gamete cells. Several deubiquitinating enzymes have been identified as important regulators of spermatogenesis and both their mutations and targeted disruption reportedly cause severe abnormalities in sperm development and fertility [19,20,21,22,23,24,25]. A recent study reported the expression of 205 genes of the ubiquitin system in gonocytes and spermatogonia [26]. Among them, 91.