An update on the ABCC transporter family in plants: many genes, many proteins, but how many functions?

The ABCC subfamily of the ATP binding cassette (ABC) transporters, which were formerly known as multidrug resistance-related proteins (MRPs), consists of closely related members found in all eukaryotic organisms. Although more than a decade of intensive research has elapsed since the first MRP protein was functionally characterised in Arabidopsis thaliana, knowledge of this particular transporter family is still limited in plants. Although ABCC proteins were originally defined as vacuolar pumps of glutathione-S (GS) conjugates, evidence, as well as speculation, on their endogenous functions inside the cell ranges from detoxification and heavy metal sequestration, to chlorophyll catabolite transport and ion channel regulation. The characterisation of knockout mutants in Arabidopsis has been pivotal for elucidation of different roles of ABCC transporters. However, a functional annotation for the majority of these transport proteins is still lacking, even in this model plant. On the one hand, this problem seems to be caused by functional redundancy between family members, which might lead to physiological complementation by a highly homologous gene in the mutant lines. On the other hand, there is growing evidence that the functional diversity of ABCC genes in Arabidopsis and other plants is far greater than previously assumed. For example, analysis of microarray expression data supports involvement of ABCC transporters in the response to biotic stress: particular changes in ABCC transcript levels are found, which are pathogen-specific and evoke distinct signalling cascades. Current knowledge about plant ABCC transporters indicates that novel and unexpected functions and substrates of these proteins are still waiting to be elucidated.