Figure 7. Subunit substitution transport assays.

(a) Transport was measured with subunit substitution of HiSiaPQM with the fused SiaPQM from A. actinomycetemcomitans (Aa) and the non-fused SiaPQM from P. profundum (Pp). Transport activity was measured in the presence of a membrane potential and a Na⁺ gradient. The mean activity is shown as bars with SEM error from at least three technical replicates (n = 3 or 4). (b) Electrostatic surface comparison of the putative SiaQM interaction surfaces of HiSiaP, AaSiaP and PpSiaP. The SiaP proteins of the two fused systems have a greater area of negatively charged residues (red, circled) at the N-terminal lobe than in the non-fused system.