Serine 403 Phosphorylation of p62/SQSTM1 Regulates Selective Autophagic Clearance of Ubiquitinated Proteins

Abstract

Selective macroautophagy (autophagy) of ubiquitinated protein is implicated as a compensatory mechanism of the ubiquitin-proteasome system. p62/SQSTM1 is a key molecule managing autophagic clearance of polyubiquitinated proteins. However, little is known about mechanisms controlling autophagic degradation of polyubiquitinated proteins. Here, we show that the specific phosphorylation of p62 at serine 403 (S403) in its ubiquitin-associated (UBA) domain increases the affinity between UBA and polyubiquitin chain, resulting in efficiently targeting polyubiquitinated proteins in "sequestosomes" and stabilizing sequestosome structure as a cargo of ubiquitinated proteins for autophagosome entry. Casein kinase 2 (CK2) phosphorylates S403 of p62 directly. Furthermore, CK2 overexpression or phosphatase inhibition reduces the formation of inclusion bodies of the polyglutamine-expanded huntingtin exon1 fragment in a p62-dependent manner. We propose that phosphorylation of p62 at S403 regulates autophagic clearance of ubiquitinated proteins and protein aggregates that are poorly degraded by proteasomes.

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

“…The UBA domain is phosphorylated on Ser-403 by Casein Kinase II. This modification increases the domain's affinity for poly-ubiquitin chains and the tendency of p62/SQSTM1 to form p62 bodies/ sequestosomes, small protein inclusions that can function as signaling complexes and are cleared by autophagy (Matsumoto et al 2011). The p62/SQSTM1 protein may associate with different ubiquitin-tagged cargos and direct them to the proteasome or the lysosome depending on its posttranslational modifications: in fact, knockdown of p62/SQSTM1 impairs the degradation of some proteasome substrates, and p62/SQSTM1 can associate with the proteasome through its PB1 domain (Seibenhener et al 2004).…”

“…Mature sequestosomes are targeted to the autophagosomes by the LIR sequence on their p62/ SQSTM1 moieties and degraded in the lysosome (Pankiv et al 2007). Sequestosomes can form around already existing protein inclusions, such as mutant huntingtin aggregates (Bjorkoy et al 2005), and their formation is greatly enhanced by the phosphorylation of p62/SQSTM1 on serine 403 in the UBA domain (Matsumoto et al 2011). Interestingly, p62/SQSTM1 must oligomerize to localize the aggregates to autophagosome-forming structures and for efficient engulfment and delivery of autophagy substrates (Itakura and Mizushima 2011).…”

Peroxisomal protein PEX 13 functions in selective autophagy

“…Our observation that PEX13 did not colocalize with markers of the early autophagosome (WIPI2) or with the selective autophagic cargo (TOMM20) during basal and mitophagy conditions suggests it does not function as an autophagy receptor. Given the precedent of peroxisome‐associated proteins functioning as signaling platforms 3, 4, we speculate that PEX13 may regulate selective autophagy as a peroxisomal membrane‐associated signaling node, leading to downstream posttranslational modifications (such as ubiquitylation 28 or phosphorylation 29, 30) and activation of other yet‐to‐be identified selective autophagy regulators. Unbiased proteomics approaches will be helpful to interrogate this pathway.…”

Immune and myodegenerative pathomechanisms in inclusion body myositis

“…In IBM muscle, both p62/SQSTM1 and NBR1 are upregulated on protein and mRNA level and colocalize with phosphorylated tau in protein aggregates 222, 284. The cargo protein p62/SQSTM1 can bind Lys63‐linked ubiquitin and phosphorylation at Ser403 of p62 enhances the binding affinity of p62 to ubiquitin 285. A recent study demonstrates that aggregated p62/SQSTM1 is largely phosphorylated at Ser403 in muscles of IBM patients and Lys63‐linked ubiquitin colocalized with p62/SQSTM1 aggregates, suggesting impaired initiation of selective autophagy targeting ubiquitinated proteins 286…”