As a pathway of protein degradation and posttranslational modification [1], ubiquitin-proteasome pathway (UPP) is responsible for the degradation of 80% - 90% bulk protein in eukaryotic cell [2]. UPP is involved in a complex series of intracellular processes, such as gene expression regulation [3], cell cycle regulation, DNA damage repair, immune response [4], cell apoptosis [5] and so forth. To prevent cells from undergoing proteotoxic stress, UPP controls the levels of cellular proteome by regulating the complex series of intracellular processes [6]. UPP is a series of complex enzymatic cascade reaction and composed of ubiquitin-activating enzyme (E1), ubiquitin-conjugating enzyme (E2) and ubiquitin-ligating enzyme (E3) [7]. With the help of the three enzymes and adenosine triphosphate (ATP), UPP finishes ubiquitinating the substrate proteins. As the second enzyme of UPP, E2 is highly conserved in different species. E2 plays an extremely important role in ubiquitin (Ub) transferring and substrate specific recognition. In recent years, E3 and viral Ub become the focus of attention. There has recently been great progress in the functions of E3s and viral Ub gene. However, up to now, not much research has been performed on the functions of E2s in cells except the transferring of Ub. More and more researchers prove that UPP is involved in cellular immune reactions caused by pathogens. Interestingly, E2s have always been detected in the immune reactions. But the functions of E2s in immune reactions caused by pathogens are still unclear. This review presents an overview of the current researches on different E2s and the immune responses of cells caused by viruses and bacteria. The purpose of this review is to enhance understanding of the functions of E2s when cells suffer from the invasion of pathogens.

UPP is a main way of protein degradation in eukaryotic cells. Ubiquitination of the target protein comprises three steps. Firstly, ATP-dependent E1 activated Ub by C-terminal adenylation, followed by formation of a high-energy thioester bond. Secondly, activated Ub is then transferred from E1 to the active-site cysteine of E2 preserving the thioester bond. Third, E3 promote transfer of Ub from E2 to the target protein [8-10]. At last, the targeted proteins are transported to proteasome for degradation (Figure 1) [11]. Actually, Ub is a kind of special protein with a very long life span and itself cannot be degraded. Polyubiquitin chain can be dissociated for target proteins with the help of deubiquitinating enzymes (DUBs). The dissociated Ubs can participate in targeting next substrate protein. In previous studies, the connection between the ubiquitination and the activation of the

genes, which are involved in the cellular immune responses, has been identified. To control the activity of some key factors regarding to immune responses, cells are likely to up-regulate the expression level of genes involved in progress of protein modification, for example the cytokine TNF-α [12].

Eukaryotic genomes encode large families of E2s and eukaryotic E2 originates from the Guillardia theta nucleomorph about 2500 million years ago [2]. E2 is mainly vertically inherited through eukaryotic evolution [13]. In Guillardia theta nucleomorph, a lot of genes may have been lost during the process of evolution, and the genes only necessary for survival are retained. However, three E2 genes have been identified among 632 reference sequences [14]. Up to now, 17 kinds of E2s have been found in yeast and 13 in Arabidopsis thaliana [15]. Human genome encodes 60 - 70 E2s. The diversification of E2s is quite essential to the specificity of substrates. E2s all have a conservative UBC domain composed of 150 amino acids and within the UBC domain E2s possess a specific active site cysteine residue. The structure of E2 is comprised of an N-termianal α-helix, three C-termianal α-helix and a four-stranded antiparallel β-sheet [16]. Most of the E2s are proteins with small molecular weight [17]. Based on structural analysis, E2s could be divided into four groups: Class I only consists of the conserved UBC domain [18]; ClassⅡ has additional C-terminal extensions and Class Ⅲ has additional N-terminal extensions [19,20]; Class Ⅳ possesses both N and C-terminal extensions [21]. Different classes of E2s are very meaningful to different target proteins of UPP.

Whether bacterium or virus, the invasion of pathogens can give rise to a series of cellular immune responses. UPP is the chief pathway of cellular protein hydrolysis. Nowadays, many researches are focused on the changes of enzymes involved in UPP when the pathogens invade host cells, in order to enhance understanding of the activities of pathogens in cells. E2, a key enzyme of UPP, can directly or indirectly conjugate Ub to bulk protein. Recently, it has been identified in some studies that E2 can interact with the proteins of pathogens. The research results not only do provide evidences for the interaction between the host proteins and pathogens, but lay the foundation for the further functions study of E2s. At present, the researches of E2s are largely focused on several model organisms, such as yeast, Arabidopsis thaliana and mouse. Furthermore, some E2s researchers place emphasis on the aspect of antitumor. Using DNA microarrays, an ubiquitin-conjugating enzyme E2C gene (UBE2C) was found to be the most highly overexpressed gene in both the primary tumors and the liver metastases [22]. Current studies have shown the increase of E2 expression levels challenged to some pathogens, such as bacteria and viruses. In the review, we sum the research results about interaction between E2s and pathogens recently and discuss the functions of E2s in cellular immune responses caused by viruses and bacteria.

In this review, we focused on showing the different functions of E2s in cellular immune responses caused by viruses and bacteria. As a key component of UPP, E2 is crucial for the protein degradation and modification. Previous studies have reported that pathogens interact with cellular immune responses by targeting host E2s. Successful viruses or bacteria have evolved superior strategies to escape host defenses or exploit host biological pathways. Some kinds of E2s, to a certain extent, assist viruses or bacteria to survive and proliferate in host cells. However, the E2s perform distinct functions in cellular response. Some E2s are likely to be concerned with resistance against pathogens. By enhancing some key genes related to immune response activity, some E2s can interfere with the survival and proliferation of pathogens. Therefore, the functions of E2s family in cell are not uniform. The difference of E2s makes the research of E2 family become difficult. However, with the tireless efforts, we have obtained a great many achievements on E2s researches. The researches on E2s not only provide us the functions of E2s family, but also help us to clarify the association between pathogens and host UPP. What’s more, the association will assist us in understanding the exploitation process of metabolic substrates actualized by pathogens and help us find some potential targets to cure certain diseases.

This work was supported by Postdoctoral daily finance of Jiangsu University (No. 1143002139), and the National Basic Research Program (973) of China (No. 2012CB4604).