molecule Organic Cation Transporter OCT family Organic Cation Transport Protein Organic Cation Transporter proteins carry positively charged chemical substances containing covalently bound carbon atoms through solubility barriers, such as lipid membranes or across epithelial layers. After binding substrate, transmembrane transporters often undergo an energy-dependent conformational change that moves the substrate across the membrane with (symport) or against (antiport) concentration gradients.