The Role of E3 Ubiquitin Ligases in Chloroplast Function

Abstract

Chloroplasts are ancient organelles responsible for photosynthesis and various biosynthetic functions essential to most life on Earth. Many of these functions require tightly controlled regulatory processes to maintain homeostasis at the protein level. One such regulatory mechanism is the ubiquitin-proteasome system whose fundamental role is increasingly emerging in chloroplasts. In particular, the role of E3 ubiquitin ligases as determinants in the ubiquitination and degradation of specific intra-chloroplast proteins. Here, we highlight recent advances in understanding the roles of plant E3 ubiquitin ligases SP1, COP1, PUB4, CHIP, and TT3.1 as well as the ubiquitin-dependent segregase CDC48 in chloroplast function.

Keywords: CDC48; CHIP; COP1; E3 ligase; PUB4; SP1; TT3.1; chloroplast; homeostasis; photosynthesis; protein degradation; stress; ubiquitin.

Figure 1

Schematic representation of the mechanisms of E3 ubiquitin ligases in regulating chloroplast function. (A) SP1-mediated degradation of TOC machinery through the CHLORAD pathway. The specific subunits targeted (Toc33/34/75/120/132 and Toc159) are represented as TOC (detailed in Section 2.1). (B) Light and dark pathways of the feedback mechanism involving COP1, HY5, and ABI4 in chloroplast development (detailed in Section 2.2). (C) Role of PUB4 in chloroplast degradation (detailed in Section 2.3). (D) CHIP-mediated degradation of chloroplast protease precursor proteins in the cytosol (detailed in Section 2.4). (E) Simplified schematic of TT3.1-mediated degradation of TT3.2 under heat stress (detailed in Section 2.5). (F) CHLORAD pathway involving SP1 and CDC48 (detailed in Section 3). Ub, ubiquitin; OM, outer membrane; IM, inner membrane; PTM, plant homeodomain type transcription factor with transmembrane domains. All images were created with BioRender.com.