Mm. Stephan et al., AN EXTRACELLULAR LOOP REGION OF THE SEROTONIN TRANSPORTER MAY BE INVOLVED IN THE TRANSLOCATION MECHANISM, Biochemistry, 36(6), 1997, pp. 1322-1328
The serotonin transporter (SERT) is a member of a highly homologous fa
mily of proteins responsible for the reuptake of biogenic amines from
the synaptic cleft. We took advantage of native restriction sites in S
ERT to construct a chimeric transporter containing a small (34 amino a
cid) region of the norepinephrine transporter. The substituted region
corresponds to about half of the largest extracellular loop. This chim
era transports serotonin very slowly compared to wild type SERT. Howev
er, it binds serotonin and the cocaine analog 2 beta-carbomethoxy-3 be
ta-(4-[I-125]iodophenyl)tropane with a high affinity indistinguishable
from wild type. It has the same specificity as wild type SERT for the
antidepressants paroxetine and desipramine. The low rate of transport
does not appear to be due to poor expression, since the chimeric tran
sporter is expressed at the membrane surface at close to wild type lev
els as measured by cell surface biotinylation. These observations lead
us to conclude that, rather than playing a role in substrate or drug
binding, this region of the large extracellular loop may be involved i
n the conformational changes associated with substrate translocation i
nto the cell.