Are there any proteins not found in the brain that are affected by prions?
Yes, malformed prion proteins can affect tissues outside of the brain.

Via: https://www.merckmanuals.com/home/brain,-spinal-cord,-and-nerve-disorders/prion-diseases/overview-of-prion-diseases


  Another familial prion disease has been recently discovered. It differs from other prion diseases because it causes diarrhea and affects nerves throughout the body years before symptoms of brain malfunction develop. It is described as prion disease associated with diarrhea and autonomic neuropathy.


Via: https://www.merckmanuals.com/home/brain,-spinal-cord,-and-nerve-disorders/prion-diseases/prion-disease-associated-with-diarrhea-and-autonomic-neuropathy


  Symptoms begin when people are in their 30s. People have persistent watery diarrhea and bloating. They may lose weight. Because the nerves that control body processes are affected, people may not be able to pass urine (called urinary retention) or may lose control of their bladder (urinary incontinence). Their blood pressure may drop when they stand up, causing them to feel dizzy or light-headed (called orthostatic hypotension). People may lose sensation in their feet. Later, when people are in their 40s or 50s, mental function deteriorates, and seizures may occur.