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(1) controlling when and how often a given gene is transcribed, (2) controlling how an RNA transcript is spliced or otherwise processed

(3) selecting which mRNAs are exported from the nucleus to the cytosol

(4) regulating how quickly certain mRNA molecules are degraded, (5) selecting which mRNAs are translated into protein by ribosomes

(6) regulating how rapidly specific proteins are destroyed after they have been made; in addition, the activity of individual proteins, once they have been synthesized, can be further regulated in a variety of ways

to the cell cortex inside the cell

to extracellular matrix molecules outside the cell

to proteins on the surface of another cell

d iffusion barriers

integral proteins

lipid-anchored membrane proteins

peripheral membrane proteins

glycoprotein

Transmembrane proteins can extend across the bilayer as a single α helix, as multiple α helices, or as a rolled-up β sheet (called a β barrel). These transmembrane proteins are amphipathic, having both hydrophobic and hydrophilic regions. Their hydrophobic regions lie in the interior of the bilayer, nestled against the hydrophobic tails of the lipid molecules. Their hydrophilic regions are exposed to the aqueous environment on either side of the membrane.

Some membrane proteins are anchored to the cytosolic half of the lipid bilayer by an amphipathic α helix.

Some proteins lie entirely outside the bilayer, on one side or the other, attached to the membrane by one or more covalently attached lipid groups.

Many proteins are attached to the membrane only by relatively weak, noncovalent interactions with other membrane proteins.

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