2.A.84 The Chloroplast Maltose Exporter (MEX) Family

The MEX family includes just one functionally characterized protein in Arabidopsis thaliana, the MEX1 protein (Niittylä et al., 2004). It is synthesized as a precursor and processed to give the mature protein. The protein has 415 aas with an N-terminal hydrophilic domain of about 120 residues followed by the hydrophobic domain with 9-10 putative TMSs. It is essential for the conversion of starch to sucrose in Arabidopsis leaves at night and catalyzed the predominant route of carbohydrate export from chloroplasts at night. Members of the family are only found in plants (e.g., rice and potatoes). Mutants of A. thaliana lacking the MEX1 protein grow poorly and have a reduced chlorophyll content.

The reaction catalyzed by MEX1 is:

maltose (in the chloroplast) → maltose (cytoplasm).



Niittylä, T., G. Messerli, M. Trevisan, J. Chen, A.M. Smith, and S.C. Zeeman. (2004). A previously unknown maltose transporter essential for starch degradation in leaves. Science 303: 87-89.

Reidel, E.J., R. Turgeon, and L. Cheng. (2008). A maltose transporter from apple is expressed in source and sink tissues and complements the Arabidopsis maltose export-defective mutant. Plant Cell Physiol. 49: 1607-1613.


TC#NameOrganismal TypeExample
2.A.84.1.1The chloroplast MEX1 maltose exporterPlantsMEX1 of Arabidopsis thaliana
2.A.84.1.2The apple chloroplast maltose exporter, Mex (Reidel et al., 2008).


MEX of Malus domestica (Q0GR04)


Hypothetical protein of 370 aas


Hypothetical protein of Synechococcus sp. PCC 7002 (B1XHU2)