A single serine residue controls the cation dependence of substrate transport by the rat serotonin transporter

The serotonin transporter (SERT) is a member of the Na(+)/Cl(−)-dependent neurotransmitter transporter family and constitutes the target of several clinically important antidepressants. Here, replacement of serine-545 in the recombinant rat SERT by alanine was found to alter the cation dependence of serotonin uptake. Substrate transport was now driven as efficiently by LiCl as by NaCl without significant changes in serotonin affinity. Binding of the antidepressant [(3)H]imipramine occurred with 1/5th the affinity, whereas [(3)H]citalopram binding was unchanged. These results indicate that serine-545 is a crucial determinant of both the cation dependence of serotonin transport by SERT and the imipramine binding properties of SERT.