How the nucleus copes with proteotoxic stress

Abstract

The proper folding of proteins is continuously challenged by intrinsic and extrinsic stresses, and the accumulation of toxic misfolded proteins is associated with many human diseases. Eukaryotic cells have evolved a complex network of protein quality control pathways to protect the proteome, and these pathways are specialized for each subcellular compartment. While many details have been elucidated for how the cytosol and endoplasmic reticulum counteract proteotoxic stress, relatively little is known about the pathways protecting the nucleus from protein misfolding. Proper maintenance of nuclear proteostasis has important implications in preserving genomic integrity, as well as for aging and disease. Here, we offer a conceptual framework for how proteostasis is maintained in this organelle. We define the particular requirements that must be considered for the nucleus to manage proteotoxic stress, summarize the known and implicated pathways of nuclear protein quality control, and identify the unresolved questions in the field.

Figure 1. The proteostasis network maintains a functional proteome

Molecular chaperones and the ubiquitin-proteasome system (UPS) cooperate in pathways of protein folding, refolding, disaggregation, and degradation. At the cellular level, the accumulation of protein aggregates is also managed by autophagic degradation, mitotic clearance, and physical sequestration pathways.

Figure 2. The integrities of the nuclear envelope, nuclear pore complexes, and transport pathways are critical for preserving proteostasis in the nucleus

These three factors are speculated to protect the nuclear proteome by (A) restricting access of aggregation-prone nascent polypeptide synthesis and folding processes; (B) transporting protein quality control (PQC) machineries into the nucleoplasm to establish the nuclear proteostasis network; and (C) possibly sensing nuclear protein misfolding to signal for increased import of PQC components (1), as well as clearing the nucleus of misfolded or aggregated protein (2). Molecular pathways that sense and respond to nuclear protein misfolding are currently unknown. ONM, outer nuclear membrane; INM, inner nuclear membrane; HSP, heat shock protein/chaperone; E3, E3 ubiquitin ligase.

Figure 3. Cytoplasmic chaperones shuttle into the nucleus under various conditions

(A) Several cytoplasmic chaperones contain nuclear-targeting peptide signals and can be imported into the nucleus by RanGTP-mediated mechanisms in ambient growth conditions. (B) Cytoplasmic chaperones shuttle into the nucleus upon acute environmental stress. For Hsp70, this transport occurs independently of RanGTP and is mediated by the Hikeshi protein. (C) Chaperones are found associated with nuclear aggregates composed of disease-associated proteins, i.e. chronic nuclear misfolding stress. It is thus far unclear how these chaperones are transported into the nucleus under these conditions.

Figure 4. The nuclear UPS in budding yeast

(A) Misfolded substrates within the nucleus are polyubiqitinated for degradation by nuclear resident E3 ligases San1 and Slx5-Slx8, and the integral membrane protein Doa10. San1 also polyubiquitnates cytoplasmic misfolded substrates targeted to the nucleus. The E3 ligase Ubr1 and its S. pombe homolog Ubr11 (not shown) also mediate degradation of proteins that reside in or sample the nucleus. The different colored misfolded proteins represent substrates with different misfolded moieties. (B) Proteasomes are imported into the nucleus by RanGTP-dependent transport pathways. Several individual 20S subunits possess nuclear localization-like signals and are transported by the importins; the assembled 20S core particle can also be imported by the proteasomal activator Blm10. Cut8/Sts1 retains proteasomes in the nucleoplasm. Note that the role for nuclear proteasomes in nuclear proteostasis has not been formally established. See text for a discussion on nuclear UPS in higher eukaryotes.

Figure 5. Hypothetical mechanisms for clearance of nuclear aggregates

(A) Nuclear aggregates may be cleared during mitosis through asymmetric inheritance, as in yeast (top panel). In metazoans, the dissolution of the nuclear envelope may expose nuclear aggregates to cytoplasmic PQC machineries (bottom panel). (B) Large nuclear aggregates have been proposed to actively transport through the nuclear envelope to the cytoplasm for disaggregation or degradation, analogous to nuclear egress pathways of Herpesvirus and mRNA-protein granules; adapted from a model proposed by Schlieker and colleagues [125]. These models require experimental validation.