The two business assets not mentioned in the previous answer are Cherry Popper's Ice Cream and Pole Position. You don't need to complete all of them...just six...but Printworks must be one of the six that you complete. The "Protection Ring" asset must also be completed (by finishing "Copland")...and completion of "Sir, Yes Sir!" for Cortez is also required. The remaining missions..."All Hands On Deck" for Cortez and all of the Avery, Love Fist/Mitch Baker, Cuban/Haitian, Phil, telephone assassination, and emergency vehicle missions...as well as offroad, RC, and street racing challenges (in addition to 3 of the 4 stolen vehicle lists and any other "free roam"-related task) are only required for 100% game completion.

BTW if you lost track of which businesses are valid completed assets toward unlocking the final missions, the $ icons are there to remind you (as well as provide you with additional cash). If the business is not yet a completed asset, the icon won't be there.

Once you've completed "Sir, Yes Sir!" for Cortez and have $ icons that spawn at the mansion, Printworks, and at least 5 additional businesses...a telephone call from Ernest Kelly signals that the final missions are available by instructing you to visit Printworks (hence, completion of that business is mandatory).