An antiport protein is a type of transporter protein which allows two different molecules to pass across the cell membrane at the same time.
Transport of molecules across the membrane
Transport occurs as an exchange between the molecules located on either side of the membrane. Both molecules therefore need to be in contact with the antiport in order for it to carry them in the opposite directions.
Their transport occurs along the electrochemical gradient of the molecules in question on either side of the membrane.
Function of the antiport protein
Antiport proteins are partly responsible for facilitated diffusion of solutes across cell and organelle membranes.
The different types of facilitated diffusion transporter proteins. M is a section through the cell membrane. An antiport protein is seen in III © Zoph, Wikimedia CC by-sa 3.0