The specificity of monoclonal antibodies (2F4) raised against homopolygalacturonic acid has been extensively characterized using immunoassays. The 2F4 antibody recognizes a conformation of homopolygalacturonic acid induced by calcium ions, which corresponds probably to an intermolecular association of pectic chains in dimers such as these described by the "egg-box" model.
This antibody has been used, in immunocytochemistry, to study the distribution of pectins in walls of suspension cultured carrot cells, compact and friable sugar-beet calli and carrot root apices. Labelings show clearly that low methyl-esterified pectins are essentially restricted to the expanded regions of middle lamellae at junction zones and intercellular spaces, where cell adhesion is weak. These polymers are not located on primary walls and on portions of middle lamellae between closely appressed cells, which are found to contain either highly methyl-esterified pectins (carrot) or largely methyl- and acetyl-esterified pectins (sugar-beet). These structural observations do not support the widely accepted concept that polygalacturonic acid sequences, dimerized through calcium ions according to the "egg-box" model, play a major role in cell wall cohesion and intercellular adhesion at the middle lamella.