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Iron-Sulfur Cluster Biosynthesis in Chloroplasts

Iron-sulfur (Fe-S) clusters are some of the most common and functionally diverse types of prosthetic groups found in living organisms. Although Fe-S clusters are derived from two of the most abundant elements on earth, they are not formed spontaneously in cells. Instead, they arise by controlled biosynthesis requiring an intricate interplay of numerous proteins. SufE is one of the proteins that participate in this process. It activates NifS-like proteins, which are involved in cysteine desulfurization, the first step in Fe-S cluster biosynthesis.

SufE2 and SufE3 localize to chloroplasts.

In this Paper of the Week, Narayana Murthy U. M. and colleagues characterized two new SufE proteins (SufE2 and SufE3) from Arabidopsis thaliana chloroplasts. They found that purified recombinant SufE2 activates the cysteine desulfurase activity of CpNifS. Because its expression is flower-specific and high in pollen, the authors hypothesize that SufE2 has a specific function in pollen Fe-S cluster biosynthesis. SufE3 contains two domains, one SufE-like and one with similarity to bacterial quinolinate synthase, NadA, which catalyzes a critical step in NAD biosynthesis. SufE3 displays both SufE activity and quinolinate synthase activity. The full-length protein carries a highly oxygen-sensitive (4Fe-4S) cluster in its NadA domain, suggesting that it functions as the NadA enzyme of Arabidopsis and, as such, is involved in NAD biosynthesis. The novelty of this observation resides in this being the first example of a SufE dedicated to and essential for the biogenesis of a single target Fe-S cluster on the chloroplast quinolinate synthase in a highly challenging environment.

FOOTNOTES

See referenced article, J. Biol. Chem. 2007, 282, 18254-18264

Assembling the 20 S Proteasome

The 20 S proteasome is a macromolecular complex that degrades short-lived regulatory proteins and abnormal and misfolded proteins. The 700-kDa complex is composed of 28 subunits: 14 -type and 14 -type subunits. These subunits are arranged in a cylindrical architecture, consisting of two outer -type subunit rings embracing two central -type subunit rings. The formation of the 20 S proteasome is a complex process that involves a cascade of folding, assembly, and processing events. To date, the understanding of the assembly pathway is incomplete, due to the experimental challenges of capturing short-lived intermediates.

Two possible assembly scenarios for the Rhodococcus 20 S proteasome.

In this Paper of the Week, Michal Sharon and colleagues use a real-time mass spectrometry approach to capture transient species along the assembly pathway of the 20 S proteasome from Rhodococcus erythropolis. By recording mass spectra throughout the reaction time course they were able to monitor the formation of an early /-heterodimer as well as an unprocessed half-proteasome particle. Formation of the mature holoproteasomes occurred in concert with the disappearance of half-proteasomes. They were also able to determine in great detail the cleavage sites within the -subunit propeptides during the different assembly states. In addition to providing valuable insight into the 20 S proteasome assembly process, these results highlight the power of this method for structural biology and biogenesis studies of macromolecular complexes in particular.

FOOTNOTES

See referenced article, J. Biol. Chem. 2007, 282, 18448-18457

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