Peripheral membrane protein
Peripheral membrane proteins are proteins
that adhere only loosely to the biological membrane
with which they are associated. These molecules do not span the lipid bilayer
core of the membrane, but attach indirectly, typically by binding to integral membrane proteins. Therefore the so-called regulatory protein subunits of many ion channels and transmembrane receptors, for example, may be defined as peripheral membrane proteins. These proteins, in contrast to integral membrane proteins, tend to collect in the water-soluble fraction during protein purification.
An exception to this rule is that cells may attach an "anchor" moiety, such as the fatty-acid anchor of lipid anchored proteins, which makes the purification properties of these proteins the same as integral membrane proteins, although researchers tend not to classify them as such.