2.A.70 The Malonate:Na+ Symporter (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) and MadM (255 aas) 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 (gbAB017138) which is encoded within the malonate decarboxylase gene cluster of this organism (Chohnan et al., 1999). These proteins apparently comprise a new family of secondary carriers as PSI-BLAST searches do not reveal that they are members of other established families. The NapA Na⁺/H⁺ antiporter of Enterococcus hirae (spP26235; CPA2 family, TC #2.A.37) appears to be distantly related to MadM.

The transport reaction catalyzed by MadLM is:

H⁺ malonate (out) + Na⁺ (out) ⇌ H⁺ malonate (in) + Na⁺ (in).