UBA1: At the Crossroads of Ubiquitin Homeostasis and Neurodegeneration

Abstract

Neurodegenerative diseases are a leading cause of disability and early death. A common feature of these conditions is disruption of protein homeostasis. Ubiquitin-like modifier activating enzyme 1 (UBA1), the E1 ubiquitin-activating enzyme, sits at the apex of the ubiquitin cascade and represents an important regulator of cellular protein homeostasis. Critical contributions of UBA1-dependent pathways to the regulation of homeostasis and degeneration in the nervous system are emerging, including specific disruption of UBA1 in spinal muscular atrophy (SMA) and Huntington's disease (HD). In this review we discuss recent findings that put UBA1 at the centre of cellular homeostasis and neurodegeneration, highlighting the potential for UBA1 to act as a promising therapeutic target for a range of neurodegenerative diseases.

Figure 1

Domains of Ubiquitin-Like Modifier Activating Enzyme 1 (UBA1) and Genetic Variants Identified in X-Linked Spinal Muscular Atrophy (XL-SMA). The N-terminal half of UBA1 comprises an inactive adenylation domain (IAD) that surrounds the first catalytic cysteine half-domain (FCCD). The C-terminal half of UBA1 comprises an active adenylation domain (AAD) that surrounds the second catalytic cysteine half-domain (SCCD). The SCCD contains the reactive Cys residue that binds ubiquitin. The C-terminal ubiquitin fold domain (UFD) allows UBA1 to bind to E2 enzymes. When UBA1 is folded into its 3D structure, the FCCH and SCCH and the IAD and AAD are directly adjacent to each other (Box 1). The Met residue at position 40 provides an alternative translational start site that leads to the expression of the UBA1b isoform of the protein. The UBA1a isoform-specific N-terminal sequence contains a nuclear localization signal (NLS) and Ser residues that can be phosphorylated (P residues). Mutations in UBA1 that have been shown to cause XL-SMA cluster in exon 15 of the protein. The domain structure in this figure is based on yeast and mouse UBA1 structural analysis. The specific amino acid positions in the figure are based on the mouse UBA1 sequence, as the mouse and human UBA1 protein sequences are more than 95% identical. The amino acids that are mutated in XL-SMA as well as the amino acid sequences that surround the borders of the various domains are all perfectly conserved between mouse and human UBA1 sequences.

Figure 2

Key Figure: Contribution of Ubiquitin-Like Modifier Activating Enzyme 1 (UBA1) to Neurodegenerative Disease In spinal muscular atrophy (SMA), UBA1 levels are suppressed throughout the neuromuscular system and decrease further as the disease progresses. In combination with low levels of full-length survival motor neuron protein (SMN) (due to deletion of one copy of SMN1), this leads to motor neuron degeneration as well as a range of systemic pathologies. In Huntington's disease (HD), UBA1 activity as well as expression decreases with advancing age in a tissue-specific manner. This influences the aggregation of mutant huntingtin (HTT) protein, specifically in the brain and neuronal nuclei, which is associated with neuronal degradation. In other late-onset neurodegenerative disorders such as Parkinson's disease (PD), Alzheimer's disease (AD), and amyotrophic lateral sclerosis (ALS), UBA1 expression or activity could also decrease with age for various reasons (such as mislocalization or protein sequestering). In combination with a range of genetic and environmental risk factors that have been associated with these diseases, this could also lead to degeneration of specific subpopulations of neurons and/or affect the protein aggregation that is observed in these disorders.

Figure I

Simplified Representation of the E1–E2–E3 Ubiquitin Activation Pathway. In an ATP- and Mg²⁺-dependent reaction, ubiquitin-like modifier activating enzyme 1 (UBA1) activates ubiquitin and transfers it to one of ∼40 E2 conjugating enzymes. Subsequently, one of hundreds of E3 ligases facilitates the transfer of ubiquitin from the E2 enzyme to specifically mono- or polyubiquitylate a protein substrate. In addition, HECT E3 ligases are able to directly bind ubiquitin and thereby directly ubiquitylate protein substrates.