Washington University in St. Louis

Senior-Author Publication by Monica Sala-Rabanal, Nichols’ Lab

Congratulations to Monica Sala-Rabanal and Colin Nichols on their publication in the Journal of Biological Chemistry!

Li, DC, Nichols, CG, Sala-Rabanal, M  (2015)  Role of a Hydrophobic Pocket in Polyamine Interactions with the Polyspecific Organic Cation Transporter OCT3.  J Biol Chem  [E-published September 24] doi: 10.1074/jbc.M115.668913

Abstract:  Organic Cation Transporter 3 (OCT3, SLC22A3) is a polyspecific, facilitative transporter expressed in astrocytes and in placental, intestinal and blood-brain barrier epithelia, and thus elucidating the molecular mechanisms underlying OCT3 substrate recognition is critical for the rational design of drugs targeting these tissues. The pharmacology of OCT3 is distinct from that of other OCTs, and here we investigated the role of a hydrophobic cavity tucked within the translocation pathway in OCT3 transport properties. Replacement of an absolutely conserved Asp by charge reversal (D478E), neutralization (D478N), or even exchange (D478E) abolished MPP+ uptake, demonstrating this residue to be obligatory for OCT3-mediated transport. Mutations at non-conserved residues lining the putative binding pocket of OCT3 to the corresponding residue in OCT1 (L166F, F450L, and E451Q) reduced the rate of MPP+ transport, but recapitulated the higher-sensitivity pharmacological profile of OCT1. Thus, interactions of natural polyamines (putrescine, spermidine, spermine) and polyamine-like potent OCT1 blockers (1,10-diaminodecane, decamethonium, bis-triethylaminodecane, and 1,10 bis-quinuclidinedecane) with wild-type OCT3 were weak, but were significantly potentiated in the mutant OCT3s. Conversely, a reciprocal mutation in OCT1 (F161L) shifted the polyamine-sensitivity phenotype towards that of OCT3. Further analysis indicated that OCT1 and OCT3 can recognize essentially the same substrates, but the strength of substrate-transporter interactions is weaker in OCT3, as informed by the distinct makeup of the hydrophobic cleft. The residues identified here are key contributors to both the observed differences between OCT3 and OCT1 and to the mechanisms of substrate recognition by OCTs in general.

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