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Structural and Functional Characterization of Primary Cell Wall Cellulose Synthase Homotrimers in Plants


Core Concepts
Primary cell wall cellulose synthase (CesA) isoforms assemble into catalytically active homotrimeric complexes that can interact with each other, leading to synergistic cellulose biosynthesis.
Abstract
The content provides a detailed structural and functional characterization of primary cell wall cellulose synthase (CesA) isoforms from soybean (Glycine max). Key highlights: The three primary cell wall CesA isoforms (CesA1, CesA3, and CesA6) purify as catalytically active homotrimeric complexes, similar to secondary cell wall CesAs. The homotrimers exhibit differences in the positioning of transmembrane helix 7, creating a large lateral opening in the cellulose translocation channel. In vitro, homotrimers of different CesA isoforms can interact with each other, independent of the N-terminal domains. This interaction leads to synergistic cellulose biosynthesis. The class-specific region (CSR) of CesAs is proposed to mediate the inter-isoform interactions, potentially by providing distinct binding sites for different isoforms. The results support a model where cellulose synthase complexes (CSCs) in plants are assembled from homotrimers of different CesA isoforms, rather than heterotrimers of the same isoforms.
Stats
"Cellulose biosynthesis in the presence of 1.4, 0.5, and 2.3 mM UDP-Glc for GmCesA1, 3, and 6, respectively, and the indicated increasing concentrations of UDP." "CesA1 has an apparent Vmax of approximately 0.1 nmol/(sec mg), about four and five times higher compared to CesA3 and CesA6, respectively." "The affinities for substrate binding range from 1.4 mM for CesA1 to 0.6 and 2.4 mM for CesA3 and CesA6, respectively." "The experimental activity of combining all three CesA isoforms is about 3-fold above the theoretical value."
Quotes
"Contrasting secondary cell wall CesAs, a peripheral position of the C-terminal transmembrane helix creates a large, lipid-exposed lateral opening of the enzymes' cellulose-conducting transmembrane channels." "Our data suggest that cross-isoform interactions are mediated by the class-specific region, which forms a hook-shaped protrusion of the catalytic domain at the cytosolic water-lipid interface." "Combined, our structural and biochemical data favor a model by which homotrimers of different CesA isoforms assemble into a microfibril-producing CSC."

Deeper Inquiries

What are the potential regulatory mechanisms that control the assembly of different CesA isoform homotrimers into cellulose synthase complexes (CSCs) in the plant cell membrane

The assembly of different CesA isoform homotrimers into cellulose synthase complexes (CSCs) in the plant cell membrane is likely regulated by several mechanisms. One potential regulatory mechanism could involve the interactions between the class-specific regions (CSRs) of the CesA isoforms. The CSR domains, which are integrated into the peripheral extensions of the catalytic domain, may play a crucial role in mediating the interactions between different isoforms. These interactions could be facilitated by specific binding sites within the CSR domains, allowing for the formation of heterotrimeric complexes. Additionally, post-translational modifications or regulatory proteins could modulate the interactions between CesA isoforms, influencing the composition and stability of CSCs in the membrane. The presence of unidentified ligands coordinated by the PCR domains at the particle's symmetry axis could also contribute to the regulation of CesA assembly into functional complexes.

How do the structural differences between primary and secondary cell wall CesAs, such as the positioning of transmembrane helix 7, impact the organization and function of CSCs

The structural differences between primary and secondary cell wall CesAs, such as the positioning of transmembrane helix 7, have significant implications for the organization and function of CSCs. In primary cell wall CesAs, the displacement of TM helix 7 creates a large lateral window in the cellulose secretion channel, exposing the translocating nascent cellulose polymers to the lipid bilayer. This structural feature could impact the alignment of the nascent glucan chains into protofibrils and influence the overall architecture of the cellulose microfibrils. The positioning of TM helix 7 at the periphery of the trimer and the presence of the lateral window suggest a unique mechanism for cellulose translocation and organization in primary cell wall CSCs compared to secondary cell wall CSCs. These structural differences likely contribute to the distinct properties and functions of primary and secondary cell wall cellulose synthase complexes.

Could the synergistic cellulose biosynthesis observed in vitro be leveraged to engineer more efficient cellulose production in plants or other organisms

The synergistic cellulose biosynthesis observed in vitro, where the combined activities of different CesA isoforms exceed the theoretical additive activities, presents an exciting opportunity for engineering more efficient cellulose production in plants or other organisms. By leveraging the synergistic effects of different CesA isoforms, it may be possible to enhance cellulose biosynthesis rates and overall yield. Strategies could involve optimizing the composition of CesA isoforms in plant cells to promote the formation of heterotrimeric complexes with increased catalytic activity. Additionally, understanding the regulatory mechanisms that govern isoform interactions and synergistic effects could inform the development of novel biotechnological approaches for enhancing cellulose production in industrial settings. This could have implications for various applications, including biofuel production, biomaterials development, and sustainable agriculture.
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