The structure of the Moco carrier protein from Rippkaea orientalis

Area of Science:

• Biochemistry
• Structural Biology
• Microbiology

Background:

• The molybdenum cofactor (Moco) is essential for most molybdenum-dependent enzymes, but its cellular transport and storage mechanisms remain unclear.
• Moco is sensitive to oxidation and typically remains bound to proteins, necessitating carrier proteins for stability and function.
• A Moco carrier protein (MCP) from Chlamydomonas reinhardtii is known to bind and protect Moco, suggesting a role in its transport and storage.

Purpose of the Study:

• To investigate the presence and structure of Moco carrier proteins (MCPs) in prokaryotes, specifically cyanobacteria.
• To characterize a putative MCP from the cyanobacterium Rippkaea orientalis (RoMCP) for insights into Moco handling.
• To elucidate the structural basis for Moco binding and transport by prokaryotic MCPs.

Main Methods:

• Recombinant production and crystallization of Rippkaea orientalis MCP (RoMCP).
• X-ray crystallography for determining the three-dimensional structure of RoMCP.
• Computational docking experiments to model the interaction between RoMCP and Moco.

Main Results:

• RoMCP exhibits a Rossmann-fold topology and a homotetrameric quaternary structure, similar to the algal MCP.
• A positively charged crevice within each RoMCP protomer was identified, capable of binding chloride ions and potentially Moco.
• Computational docking suggested a plausible binding mode for Moco within the RoMCP structure.

Conclusions:

• Prokaryotic cyanobacteria possess MCPs structurally analogous to those in eukaryotes, indicating conserved Moco transport mechanisms.
• RoMCP's structure provides a potential model for Moco binding and transport, involving a specific charged crevice.
• This study expands the understanding of Moco homeostasis beyond eukaryotes and highlights the role of MCPs in prokaryotic systems.