E3 ubiquitin ligases and abscisic acid signaling

The ubiquitin proteasome system is involved in the regulation of nearly every aspect of plant growth and development. Protein ubiquitination involves the covalent attachment of ubiquitin to target proteins through a cascade catalyzed by three enzymes known as E1, E2 and E3. E3s are of particular interest as they confer substrate specificity during ubiquitination through their diverse substrate recognition domains. Recently, a number of E3s have been identified that actively participate in abscisic acid hormone biology, including regulation of biosynthesis, de-repression or activation of abscisic acid response and degradation of signaling components. In this review, we summarize recent exciting studies of the different types of E3s that target specific mediators of abscisic acid signaling or affect the plants response to the hormone.Key words: abscisic acid, E3 ubiquitin ligase, proteasome, ubiquitinationPost-translational control of protein degradation by the ubiquitin proteasome system (UPS) is a highly regulated process essential for the proper growth and development of all eukaryotes through removing abnormal proteins and most short-lived regulatory proteins.^1,2 Plants utilize the UPS to alter their proteome to mediate cellular changes required for growth, development and responses to biotic and abiotic stress. Plants also rely a great deal on hormones to induce changes in growth and development in response to a wide range of environmental stimuli. Hormone biosynthesis, perception, signaling and response can be exquisitely regulated through modulating protein levels via the UPS. Regulation of the abscisic acid (ABA) signaling pathway, like auxin, gibberellin, jasmonate and ethylene, have been linked to UPS components with the application of biochemical, genetic and genomic approaches.^3–5 Although some aspects of ABA signaling have been elucidated, the involvement of the UPS, especially E3 ubiquitin ligases, help us gain further insight into the entire network of ABA signal transduction. In this review we focus on recently identified E3s that play a variety of roles in ABA signaling. A number of articles are available that provide a comprehensive review of the role of E3 ligases in the biosynthesis, perception and signaling by other hormones such as auxin and ethylene.^3–5     

Regulation of apoptosis by E3 ubiquitin ligases in ubiquitin proteasome system

Apoptosis is an organised ATP‐dependent programmed cell death that organisms have evolved to maintain homoeostatic cell numbers and eliminate unnecessary or unhealthy cells from the system. Dysregulation of apoptosis can have serious manifestations culminating into various diseases, especially cancer. Accurate control of apoptosis requires regulation of a wide range of growth enhancing as well as anti‐oncogenic factors. Appropriate regulation of magnitude and temporal expression of key proteins is vital to maintain functional apoptotic signalling. Controlled protein turnover is thus critical to the unhindered operation of the apoptotic machinery, disruption of which can have severe consequences, foremost being oncogenic transformation of cells. The ubiquitin proteasome system (UPS) is one such major cellular pathway that maintains homoeostatic protein levels. Recent studies have found interesting links between these two fundamental cellular processes, wherein UPS depending on the cue can either inhibit or promote apoptosis. A diverse range of E3 ligases are involved in regulating the turnover of key proteins of the apoptotic pathway. This review summarises an overview of key E3 ubiquitin ligases involved in the regulation of the fundamental proteins involved in apoptosis, linking UPS to apoptosis and attempts to emphasize the significance of this relationship in context of cancer.     

A novel family of membrane-bound E3 ubiquitin ligases

A novel E3 ubiquitin ligase family that consists of viral E3 ubiquitin ligases (E3s) and their mammalian homologues was recently discovered. These novel E3s are membrane-bound molecules that share the secondary structure and catalytic domain for E3 activity. All family members have two transmembrane regions at the center and a RING-CH domain at the amino terminus. Forced expression of these novel E3s has been shown to reduce the surface expression of various membrane proteins through ubiquitination of target molecules. Initial examples of viral E3s were identified in Kaposi's sarcoma associated herpesvirus (KSHV) and murine gamma-herpesvirus 68 (MHV-68) and have been designated as modulator of immune recognition (MIR) 1, 2 and mK3, respectively. MIR 1, 2 and mK3 are able to down-regulate MHC class I molecule expression, and mK3 is required to establish an effective latent viral infection in vivo. The first characterized mammalian homologue to MIR 1, 2 and mK3 is c-MIR/MARCH VIII. Forced expression of c-MIR/MARCH VIII down-regulates B7-2, a co-stimulatory molecule important for antigen presentation. Subsequently, several mammalian molecules related to c-MIR/MARCH VIII have been characterized and named as membrane associated RING-CH (MARCH) family. However, the precise physiological function of MARCH family members remains as yet unknown.     

E3 ubiquitin ligases and mitosis: embracing the complexity

Faithful division of eukaryotic cells requires temporal and spatial coordination of morphological transitions, which ensures that the newly replicated copies of the genome are equally distributed into the two daughter cells during mitosis. One of the mechanisms ensuring the fidelity of mitotic progression is targeted, ubiquitin-dependent proteolysis of key regulators. E3-ubiquitin ligase complexes are crucial components in this pathway because they specifically select the relevant ubiquitination substrates. Cullin-based E3-ligases, such as Cul3, have recently emerged as crucial regulators of mitosis.     

E3 ubiquitin ligases and their control of T cell autoreactivity

A loss of T cell tolerance underlies the development of most autoimmune diseases. The design of therapeutic strategies to reinstitute immune tolerance, however, is hampered by uncertainty regarding the molecular mechanisms involved in the inactivation of potentially autoreactive T cells. Recently, E3 ubiquitin ligases have been shown to mediate the development of a durable state of unresponsiveness in T cells called clonal anergy. In this review, we will discuss the mechanisms used by E3 ligases to control the activation of T cells and prevent the development of autoimmunity.     

Building and remodelling Cullin–RING E3 ubiquitin ligases

Cullin–RING E3 ubiquitin ligases (CRLs) control a plethora of biological pathways through targeted ubiquitylation of signalling proteins. These modular assemblies use substrate receptor modules to recruit specific targets. Recent efforts have focused on understanding the mechanisms that control the activity state of CRLs through dynamic alterations in CRL architecture. Central to these processes are cycles of cullin neddylation and deneddylation, as well as exchange of substrate receptor modules to re‐sculpt the CRL landscape, thereby responding to the cellular requirements to turn over distinct proteins in different contexts. This review is focused on how CRLs are dynamically controlled with an emphasis on how cullin neddylation cycles are integrated with receptor exchange.     

Protein quality control: U-box-containing E3 ubiquitin ligases join the fold

Molecular chaperones act with folding co-chaperones to suppress protein aggregation and refold stress damaged proteins. However, it is not clear how slowly folding or misfolded polypeptides are targeted for proteasomal degradation. Generally, selection of proteins for degradation is mediated by E3 ubiquitin ligases of the mechanistically distinct HECT and RING domain sub-types. Recent studies suggest that the U-box protein family represents a third class of E3 enzymes. CHIP, a U-box-containing protein, is a degradatory co-chaperone of heat-shock protein 70 (Hsp70) and Hsp90 that facilitates the polyubiquitination of chaperone substrates. These data indicate a model for protein quality control in which the interaction of Hsp70 and Hsp90 with co-chaperones that have either folding or degradatory activity helps to determine the fate of non-native cellular proteins.