Fluorescence Microscopy Study of the Intracellular Sulfur Globule Protein SgpD in the Purple Sulfur Bacterium Allochromatium vinosum

Abstract

When oxidizing reduced sulfur compounds, the phototrophic sulfur bacterium Allochromatium vinosum forms spectacular sulfur globules as obligatory intracellular–but extracytoplasmic–intermediates. The globule envelope consists of three extremely hydrophobic proteins: SgpA and SgpB, which are very similar and can functionally replace each other, and SgpC which is involved in the expansion of the sulfur globules. The presence of a fourth protein, SgpD, was suggested by comparative transcriptomics and proteomics of purified sulfur globules. Here, we investigated the in vivo function of SgpD by coupling its carboxy-terminus to mCherry. This fluorescent protein requires oxygen for chromophore maturation, but we were able to use it in anaerobically growing A. vinosum provided the cells were exposed to oxygen for one hour prior to imaging. While mCherry lacking a signal peptide resulted in low fluorescence evenly distributed throughout the cell, fusion with SgpD carrying its original Sec-dependent signal peptide targeted mCherry to the periplasm and co-localized it exactly with the highly light-refractive sulfur deposits seen in sulfide-fed A. vinosum cells. Insertional inactivation of the sgpD gene showed that the protein is not essential for the formation and degradation of sulfur globules.

The Complex Interplay of Sulfur and Arsenic Bioenergetic Metabolisms in the Arsenic Geochemical Cycle

“…They can be covered with an organic layer which can differ from one organism to another: polysaccharides and proteins in extracellular globules from C. tepidum and Thioalkalivibrio versutus (Marnocha et al 2016;Liu et al 2022); specific proteins (called Sgps) in the intracellular Al. vinosum ones (Kumpel et al 2023). The process for the mobilization and metabolization of these globules, with SO4 2as the most common final product, as well as the process of excretion and further uptake, are still unclear (see Marnocha et al 2016).…”