Arabidopsis thaliana germin-like proteins: common and specific features point to a variety of functions.

Germin-like proteins (GLPs) are ubiquitous plant proteins encoded by diverse multigene families. It is not known whether they share germin's unusual biochemical properties and oxalate oxidase activity. Using specific antibodies, we have studied three GLPs (AtGER1. AtGER2 and AtGER3) in Arabidopsis thaliana (L.) Heynh. as well as in transgenic tobacco (Nicotiana tabacum L.) plants overexpressing these proteins. Like wheat ( Triticum aestivum L.) germin, these Arabidopsis GLPs are associated with the extracellular matrix (ECM) and they also seem to exist as two glycosylated isoforms. However, none of them is an oxalate oxidase. Although GLPs display several conserved features, each has its specific characteristics. Both AtGER2 and AtGER3 are oligomeric proteins that share germin's resistance to pepsin and to dissociation by heat and SDS. In contrast, AtGER1 seems to exist as a monomer. The GLPs may interact with the ECM in a variety of ways, since each is efficiently extracted by different conditions. In addition, germins and GLPs all bind Cibacron Blue, a dye often but not exclusively used for the purification of enzymes having nucleotide cofactors. In the case of AtGER2, binding to the dye is so tight that it almost allows a one-step purification of this protein. The variety of sequences, expression patterns and biochemical features indicates that GLPs could be a class of receptors localized in the ECM and involved in physiological and developmental processes as well as stress response.