For plant cell walls, opposites attract, according to Maura Cannon (University of Massachusetts, Amherst, MA), Marcia Kieliszewski (Ohio University, Athens, OH), and colleagues. Their results suggest that a positively charged protein network forms the frame to which negatively charged carbohydrates attach to make the mature wall.
Previously, the authors characterized a mutation that prevents normal development of the new cell wall that separates cells at the end of plant mitosis. In the current work, they identified the mutated gene as AtEXT3, one of 20 cell wall extensins in Arabidopsis.
As a new cell wall forms at the end of mitosis, AtEXT3-carrying Golgi vesicles fuse along the plane between daughter cells. Using atomic force microscopy in vitro, the authors showed that AtEXT3 formed a meshwork by cross-linking its multiple, repeating tyrosine residues. The authors propose that the highly periodic primary structure of AtEXT3 helps align copies of itself for cross-linking within the plane between daughter cells.
The protein's many lysines give it a strong positive charge—perfect for attracting cell wall sugars such as pectin. “Since pectins are strongly negatively charged,” Cannon says, “we propose that the network of the cell wall forms by extensin laying down a framework on which pectins can assemble.” The pectins and extensins bond through an acid–base reaction, and pectins themselves can also form linkages with each other.