A<unk>  (or simply transporter) is a protein involved in the movement of ions, small molecules, or macromolecules, such as another protein across a biological membrane.  Transport proteins are integral membrane proteins; that is they exist within and span the membrane across which they transport substances.  The proteins may assist in the movement of substances by facilitated diffusion or active transport.
Membrane transport protein
Transmembrane protein
Membrane fusion protein
Ion transporter
Ion channel
Carrier protein
Voltage-gated ion channel
Protein precursor