A membrane protein is a protein molecule (or assembly of molecules) that is either embedded in or weakly attached to a biological membrane, especially the plasma membrane. Proteins that are sufficiently embedded to remain with the membrane during the initial steps of biochemical purification are called integral membrane proteins whereas peripheral membrane proteins separate into the water-soluble phase.

A possible exception is that if the latter are anchored (see e.g. lipid anchored protein), the proteins will remain with the membrane during purification, although researchers are still likely to describe them as peripheral.

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