Ubiquitin homeostasis: from neural stem cell differentiation to neuronal development.

Abstract

Ubiquitin (Ub) is a well-known eukaryotic protein that exerts diverse signaling functions inside the cells, including targeting its substrates for proteasomal degradation (Hershko and Ciechanover, 1998). Tagging substrates with Ub or ubiquitination occurs via the action of three sequential enzymes called Ub-activating enzymes (E1), Ub conjugating enzymes (E2), and Ub ligases (E3). This entire process is called the Ub conjugation reaction and the substrate can be either monoubiquitinated or polyubiquitinated, which results in different outcomes for the target substrate. Inside the cells, Ub is in dynamic equilibrium between “free Ub” and “conjugated Ub” (Komander et al., 2009; Park and Ryu, 2014). Free Ub refers to a readily available form of Ub for the conjugation reaction, while conjugated Ub refers to single or multiple Ub attached to target substrates. Thus, it needs to be deconjugated or removed from the substrate (i.e., Ub conjugates) to be used for another conjugation reaction. The free Ub pool is generally maintained by de novo Ub synthesis via expression of ubiquitin genes and by Ub removal or recycling from Ub conjugates via various deubiquitinating enzymes (Komander et al., 2009; Park and Ryu, 2014). In mammals, there are two different classes of ubiquitin genes, Ub-ribosomal fusion genes (Uba52 and Uba80) and polyubiquitin genes (Ubb and Ubc). The resulting Ub-ribosomal subunit or (Ub)n fusion proteins are rapidly cleaved by deubiquitinating enzymes. Thus, all four ubiquitin genes are eventually responsible for the generation of free Ub, which is virtually indistinguishable, regardless of the original encoding gene. The contribution of the four different ubiquitin genes toward the constitution of the free Ub pool varies depending on cell types, the tissue origin, or environmental conditions. Although Ub is an abundant protein inside the cells, because of its dynamic nature and the abundance of its target substrates, the maintenance of free Ub above threshold levels or Ub homeostasis is important for the ubiquitination of target substrates in a timely manner. About a decade ago, it was shown that free Ub depletion by cycloheximide treatment in yeast resulted in reduced viability with increased toxicity, which was ameliorated by Ub overexpression (Hanna et al., 2003). Since then, the cellular maintenance of the free Ub pool has been of great interest because the maintenance of free Ub is expected to be important for cellular function and survival. In 2009, the presence of unanchored free Ub chains, which are converted to monomeric free Ub by a deubiquitinating enzyme to maintain the free Ub pool, was proposed (Kimura et al., 2009). An inhibitor of the deubiquitinating enzyme also plays a role in the maintenance of adequate levels of free Ub. The overexpression of the inhibitor was shown to reduce free Ub levels and disrupt Ub homeostasis, which reduced the viability of yeast under stress conditions (Kimura et al., 2009).