Characterization of a Plastoglobule-Localized SOUL4 Heme-Binding Protein in Arabidopsis thaliana.

Venkatasalam Shanmugabalaji,Felix Kessler,Bernhard Grimm

doi:10.3389/fpls.2020.00002

Venkatasalam Shanmugabalaji, Felix Kessler + Show 1 more

Open Access

https://doi.org/10.3389/fpls.2020.00002

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Abstract

Heme plays an active role in primary plant metabolic pathways as well as in stress signaling. In this study, we characterized the predicted heme-binding protein SOUL4. Proteomics evidence suggests that SOUL4 is a component of Arabidopsis plastoglobules (PGs, chloroplast lipid droplets). SOUL4 contains heme-binding motifs and the recombinant protein is shown here to bind heme in vitro. Fluorescence-tagged SOUL4 colocalized with the specific PG marker Fibrillin1A (FBN1A) in transiently transformed Nicotiana benthamiana leaves. In addition, SOUL4 cofractionated with another PG marker Fibrillin2 (FBN2) in sucrose gradient ultracentrifugation experiments. In vitro kinase experiments revealed that SOUL4 is phosphorylated by a yet unknown chloroplast protein kinase. Our data demonstrate that SOUL4 is a bona fide PG protein and may function in heme-buffering in the chloroplast.

Highlights

Tetrapyrroles play key roles in photosynthesis, respiration, and in various other biological processes
Two major radioactively labeled proteins were detected: one with an apparent molecular mass (MM) of around 35 kDa corresponding to the expected MM of the in vitro synthesized SOUL4 preprotein and the second band with an apparent MM of around 30 kDa corresponding to the mature form of SOUL4 (Figure 1A) (Venkatasalam, 2012)
To determine whether SOUL4 colocalized with the PG marker FBN1A, the two proteins were co-expressed as CFP and YFP fusion proteins in N. benthamiana leaves by agroinfiltration

Summary

Introduction

Tetrapyrroles play key roles in photosynthesis, respiration, and in various other biological processes. Chlorophyll, heme, and phytochromobilin (PFB) are the main tetrapyrroles derived from a common biosynthetic pathway in chloroplasts (Mochizuki et al, 2010). Due to their photochemical properties, misregulation of tetrapyrrole metabolism may lead to severe photooxidative damage and cell death (Busch and Montgomery, 2015). Protoporphyrin IX serves as a common intermediate in both the chlorophyll- and heme-synthesized branches of tetrapyrrole metabolism. It is proposed that these proteins protect the cell from the cytotoxic effects and bind to heme covalently and/or noncovalently (Balestrasse et al, 2005; Espinas et al, 2012)

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