Nucleocytoplasmic Shuttling of p62/SQSTM1 and Its Role in Recruitment of Nuclear Polyubiquitinated Proteins to Promyelocytic Leukemia Bodies

Abstract

p62, also known as sequestosome1 (SQSTM1), A170, or ZIP, is a multifunctional protein implicated in several signal transduction pathways. p62 is induced by various forms of cellular stress, is degraded by autophagy, and acts as a cargo receptor for autophagic degradation of ubiquitinated targets. It is also suggested to shuttle ubiquitinated proteins for proteasomal degradation. p62 is commonly found in cytosolic protein inclusions in patients with protein aggregopathies, it is up-regulated in several forms of human tumors, and mutations in the gene are linked to classical adult onset Paget disease of the bone. To this end, p62 has generally been considered to be a cytosolic protein, and little attention has been paid to possible nuclear roles of this protein. Here, we present evidence that p62 shuttles continuously between nuclear and cytosolic compartments at a high rate. The protein is also found in nuclear promyelocytic leukemia bodies. We show that p62 contains two nuclear localization signals and a nuclear export signal. Our data suggest that the nucleocytoplasmic shuttling of p62 is modulated by phosphorylations at or near the most important nuclear localization signal, NLS2. The aggregation of p62 in cytosolic bodies also regulates the transport of p62 between the compartments. We found p62 to be essential for accumulation of polyubiquitinated proteins in promyelocytic leukemia bodies upon inhibition of nuclear protein export. Furthermore, p62 contributed to the assembly of proteasome-containing degradative compartments in the vicinity of nuclear aggregates containing polyglutamineexpanded Ataxin1Q84 and to the degradation of Ataxin1Q84.

Identification and functional analysis of Joka2, a tobacco member of the family of selective autophagy cargo receptors

“…Additionally, we export signal (NES) and two basic monopartite nuclear localization signals (NLS1 and NLS2) are located between residues 303-320, 186-189 and 264-277, respectively. 58 The three domains (PB1, ZZ and UBA) characterized in p62 can be also found in its plant homologs, though the plant proteins are generally larger ( Fig. 2B and Table S1).…”

p62/SQSTM1 at the interface of aging, autophagy, and disease

“…The primary sequence of p62/SQSTM1 contains two nuclear localization signals (NLS) and one Nuclear Export Signal (NES), and the protein shuttles between the cytoplasm and the nucleus, where it seems to help in sequestering and degrading ubiquitin-conjugated nuclear proteins (Pankiv et al 2010). It has also been proposed that p62/SQSTM1 enters the nucleus in association with binding partners containing NLS, like aPKC, and that it regulates transcription in cooperation with these binding partners (Geetha and Wooten 2002).…”

The role of ALFY in selective autophagy

“…p62 has been suggested to facilitate proteasome recruitment to these sites and thereby help the degradation of misfolded nuclear proteins. 66 The localization of ALFY to PML-NBs is dependent on p62 and vice versa. 49 If and how nuclear aggregates are degraded by autophagy is currently not known, but three different models can be proposed; (i) p62 and ALFY bind and export misfolded ubiquitinated proteins through the nuclear pores, (ii) p62 and ALFY associate to nuclear aggregates that become degraded by autophagy during mitosis when the nuclear membrane breaks down and (iii) p62 and ALFY facilitate sorting of nuclear aggregates into nuclear membrane blebs, which become pinched off and degraded by autophagy in a process similar to yeast piecemeal microautophagy of the nucleus.…”

“…p62 is mainly localized in the cytoplasm and its nuclear import is modulated by phosphorylation at or near one of its NLSs. 66 It will be interesting to elucidate whether nucleocytoplasmic shuttling of ALFY is also regulated by post-translational modifications.…”

“…48,66 PML-NBs has been associated with a broad specter of nuclear processes such as transcription, DNA repair, stress, proteolysis and apoptosis. [67][68][69] Both p62 and ALFY were found to be required for recruitment of ubiquitinated proteins to these nuclear bodies, 49 although it cannot be excluded that they rather facilitate recruitment of E3 ligases to PML-NBs for ubiquitination of misfolded proteins.…”