磷酸化p62/SQSTM1對其功能的影響

Abstract

p62/SQSTM1是一個可與泛素結合的鷹架蛋白(scaffold protein)，可參與許多訊息傳遞途徑。它可藉由IL-1或是TNF-α的活化以調控NF-κB的表現，也可經由泛素鏈結(ubiquitinantion)，調控訊息傳遞。p62是一個高度磷酸化蛋白質，且至少有18個氨基酸已被鑑定出有磷酸化。但是造成這些後修飾作用的生理意義、訊息傳遞路徑，以及對於p62的功能有何影響，皆還不清楚。p62可藉由PB1、ZZ、TB、LIR及UBA區域作為一個接引蛋白(adaptor protein)，而有些已被鑑定出的磷酸化氨基酸坐落在這些特殊區域裡，所以這些磷酸化氨基酸對於p62的接引功能可能有所影響。在這篇論文中，我們發現到S24這個氨基酸是受到PKA所調控，且這個氨基酸的磷酸化會調控p62與PKCζ的結合。而且我們也發現S24、S272、S332、S355及S366會影響LC3B的脂化作用(lipidation)和自我吞食機制(autophagy)。且從脂化作用結果，可知S272是作為一個負向的調控因子，而S355則相反為一個正向調控因子。而在功能上的研究，在經過RNAi降低內生(endogenous)的p62，再補回不被RNAi影響的野生型(WT)及突變型(mutant)的p62，可發現S24的磷酸化在MEK5-ERK5-Mef2C 訊息傳遞路徑上扮演一個正向調控的角色。

p62/SQSTM1 is a ubiquitin-binding, scaffold protein which involves in diverse signaling pathways. It may regulate NF-κB activation by IL-1, TNF-αand signaling cascades through ubiquitination. It is a highly phosphorylated protein with at least eighteen phosphorylation sites has been identified. The physiological cues and the signaling pathways leading to these posttranslational modifications and the functional consequences remain poorly understood. p62/SQSTM1 serves as an adapter protein through PB1, ZZ, TB, LIR and UBA domains. Some of the identified phosphorylation sites fall into or in the vicinity of these domains. It is likely that some of these phosphorylations may have important consequence for the adapter functions of p62. In this study, we have found that S24 is a target of PKA. Phosphorylation of p62/S24 could regulate its interaction with PKCζ. Further, phosphorylations at S24, S272, S332, S355 and S366 can affect the lipidation of LC3B and autophagy. And phosphorylation of S272 serves as a negative while S355 as a positive regulatory functions. Functional studies by RNAi knockdown of endogenous and complemented with RNAi-resistant wild-type or mutant p62 showed that S24 phosphorylation is a positive regulator in MEK5-ERK5-Mef2C signaling pathway.