Functional analysis of the COBRA-like family

The COBRA protein is found only in higher plants and algae, and has been implicated in cellulose synthesis, but its specific function remains unknown. To shed light on COBRA function we tested the ability of putative Arabidopsis COBRA orthologs to complement mutations in the COBRA gene. We expanded the previously published phylogenetic analysis and found that COBRA genes can be grouped into five clades. Representative COBRA genes from the different clades were expressed in a cobra mutant background. Surprisingly, none of the COBRA-LIKE genes was able to complement cobra, including the closest homolog which we found to bind cellulose with the same avidity as COBRA. We hypothesize that the lack of ability of the COBRA-LIKE genes to complement the cobra mutant is due to an unidentified binding interaction that is sensitive to the structure of the COBRA-LIKE proteins. However, membrane-based yeast two hybrid assays failed to identify positive interactors for COBRA.