The E3 ubiquitin ligase SP1-like 1 plays a positive role in peroxisome biogenesis in Arabidopsis.

Peroxisomes are dynamic organelles crucial for a variety of metabolic processes during the development of eukaryotic organisms, and are functionally linked to other subcellular organelles, such as mitochondria and chloroplasts. Peroxisomal matrix proteins are imported by peroxins (PEX proteins), yet the modulation of peroxin functions is poorly understood. We previously reported that, besides its known function in chloroplast protein import, the Arabidopsis E3 ubiquitin ligase SP1 (suppressor of ppi1 locus1) also targets to peroxisomes and mitochondria, and promotes the destabilization of the peroxisomal receptor-cargo docking complex components PEX13 and PEX14. Here we present evidence that in Arabidopsis, SP1's closest homolog SP1-like 1 (SPL1) plays an opposite role to SP1 in peroxisomes. In contrast to sp1, loss-of-function of SPL1 led to reduced peroxisomal ß-oxidation activity, and enhanced the physiological and growth defects of pex14 and pex13 mutants. Transient co-expression of SPL1 and SP1 promoted each other's destabilization. SPL1 reduced the ability of SP1 to induce PEX13 turnover, and it is the N-terminus of SP1 and SPL1 that determines whether the protein is able to promote PEX13 turnover. Finally, SPL1 showed prevalent targeting to mitochondria, but rather weak and partial localization to peroxisomes. Our data suggest that these two members of the same E3 protein family utilize distinct mechanisms to modulate peroxisome biogenesis, where SPL1 reduces the function of SP1. Plants and possibly other higher eukaryotes may employ this small family of E3 enzymes to differentially modulate the dynamics of several organelles essential to energy metabolism via the ubiquitin-proteasome system.

Sequence and biochemical analysis of Arabidopsis SP1 protein, a regulator of organelle biogenesis.

Peroxisomes, chloroplasts, and mitochondria are essential eukaryotic organelles that host a suite of metabolic processes crucial to energy metabolism and development. Regulatory mechanisms of the dynamics and biogenesis of these important organelles have begun to be discovered in plants. We recently showed that, aside from its previously reported role in targeting chloroplast protein import proteins, the Arabidopsis ubiquitin E3 ligase SP1 (suppressor of ppi1 locus1) negatively regulates peroxisome matrix protein import by promoting the ubiquitination and destabilization of PEX13 and possibly PEX14 and other components of the peroxisome protein import apparatus. Here, we compared protein sequence and domain structure of SP1-like proteins in Arabidopsis and their human homolog, Mitochondrial-Anchored Protein Ligase (MAPL). We further characterized SP1 protein in respect to its membrane topology and ubiquitin E3 ligase activity.