<Li> Cell adhesion molecules allow cells to identify each other and interact . For example, proteins involved in immune response . </Li> <P> Integral membrane proteins are permanently attached to the membrane . Such proteins can be separated from the biological membranes only using detergents, nonpolar solvents, or sometimes denaturing agents . One such example of this type of protein which has not been functionally characterized yet is SMIM23 . They can be classified according to their relationship with the bilayer: </P> <Ul> <Li> Integral polytopic proteins, also known as "transmembrane proteins", are integral membrane proteins that span across the membrane at least once . These proteins may have different transmembrane topology . These proteins have one of two structural architectures: <Ul> <Li> helix bundle proteins, which are present in all types of biological membranes; </Li> <Li> beta barrel proteins, which are found only in outer membranes of Gram - negative bacteria, and outer membranes of mitochondria and chloroplasts . </Li> </Ul> </Li> <Li> Integral monotopic proteins are integral membrane proteins that are attached to only one side of the membrane and do not span the whole way across . </Li> </Ul> <Li> Integral polytopic proteins, also known as "transmembrane proteins", are integral membrane proteins that span across the membrane at least once . These proteins may have different transmembrane topology . These proteins have one of two structural architectures: <Ul> <Li> helix bundle proteins, which are present in all types of biological membranes; </Li> <Li> beta barrel proteins, which are found only in outer membranes of Gram - negative bacteria, and outer membranes of mitochondria and chloroplasts . </Li> </Ul> </Li>

What are the different classes of membrane proteins