|Some proteins also have a quaternary structure - the organization of two or more polypeptide chains into a multi-subunit protein. The protein CAP is a protein that binds to DNA and has quaternary structure. Click on the picture link at the bottom of the Protein G page to see the structure of "CAP-DNA." Click to see the protein only, DNA only, and then back to protein bound to DNA. How many polypeptide chains, or subunits, does CAP have? What type of secondary structure is present in CAP?