The Malonate:Na+ Symporter (MSS) Family (TC# 2.A.70) is a group of transport proteins belonging to the CPA superfamily. These proteins are composites with constituents ranging in size from 129 to 255 amino acyl residues (aas) and exhibiting 4 to 7 transmembrane segments (TMSs). A representative list of proteins belonging to the MSS family can be found in the Transporter Classification Database.
MadLM
One constituent member of the MSS family, the monobasic malonate:Na+ symporter of Malonomonas rubra, has been functionally and genetically characterized. It consists of two integral membrane proteins, MadL (129 aas; accession number O06931) and MadM (255 aas; accession number O06932) which exhibit 4 and 7 putative transmembrane spanners, respectively. The transporter is believed to catalyze the electroneutral reversible uptake of H+-malonate with one Na+, and both subunits have been shown to be essential for activity. There is a close two-subunit homologue in Pseudomonas putida, which is encoded within the malonate decarboxylase gene cluster of this organism. The NapA Na+/H+ antiporter of Enterococcus hirae (CPA2 family; TC #2.A.37) also appears to be distantly related to MadM.
The transport reaction catalyzed by MadLM is:
H+ malonate (out) + Na+ (out) ⇌ H+ malonate (in) + Na+ (in).