The two characterized members of the CitMHS family are both citrate uptake permeases from Bacillus subtilis. CitM is believed to transport a citrate^2--Mg²⁺ complex in symport with one H⁺ per Mg²⁺-citrate while CitH apparently transports a citrate^2--Ca²⁺ complex in symport with protons (Boorsma et al., 1996; Krom et al., 2000). The cation specificity of CitM is: Mg²⁺, Mn²⁺, Ba²⁺, Ni²⁺, Co²⁺, Ca²⁺ and Zn²⁺ with an order of preference in this order. CitM is highly specific for citrate and D-isocitrate and does not transport other di- and tri-carboxylates including succinate, L-isocitrate, cis-aconitate and tricarballylate (Li and Pajor, 2002; Warner and Lolkema, 2002). For CitH, the cation specificity (in order of preference) is: Ca²⁺, Ba²⁺ and Sr²⁺ (Krom et al., 2000). The two proteins are 60% identical, contain about 400 amino acyl residues and possess twelve putative transmembrane spanners. A CitM homologue in S. mutans transports citrate conjugated to Fe²⁺ or Mn²⁺ but not Ca²⁺, Mg²⁺ or Ni²⁺ (Korithoski et al., 2005).

The CitMHS family belongs within the IT superfamily (Prakash et al., 2003; Rabus et al., 1999). Members of this family are found in Gram-positive and Gram-negative bacteria, archaea and possibly in eukaryotes. These proteins all probably arose by an internal gene duplication event.

The transport reactions catalyzed by (1) CitM and (2) CitH, respectively, are:

(1) Citrate &149; Mg (out) + nH⁺ (out) → Citrate &149; Mg (in) + nH⁺ (in)

(2) Citrate (out) + nH⁺ (out) → Citrate (in) + nH⁺

(3) Citrate &149; Ca²⁺ (out) + nH⁺ (out) → Citrate &149; Ca²⁺ (in) + nH⁺ (in)