Are there any proteins not found in the brain that are affected by prions?

Prion disease is caused when cellular prion protein (PrPc) is misfolded to form abnormal prion protein called scrapie prion protein (PrPSc). These newly formed abnormal prions can't be degraded by enzymes so they keep on accumulating and are responsible for stimulating other normal proteins to fold improperly and form protein aggregates. Accumulation of these amyloids cause diseases and these misfolded prions can't be converted back to normal prion protein.

Via: https://ghr.nlm.nih.gov/condition/prion-disease#genes

These prion proteins (PrPc) are present throughout our body but in lesser proportion as compared to brain cells and recent studies have shown the possibility of this protein aggregation the cause of diseases outside the brain,

Via: https://www.ncbi.nlm.nih.gov/pubmed/21481020

MAVS, RIP1, and RIP3 are other prion-like proteins which are found in other parts of the body. These are not responsible for causing any disease. These proteins aggregate into filamentous amyloid and lead to cell death to prevent the spreading of viral infection.

Via: https://www.nature.com/articles/s41418-018-0172-x

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.

Although it may hadn't be described, I think that nothing chemical or physical would be an impediment to affect other organs, so, said this, in my opinion they could exist.

I found this one affecting outside the brain https://www.nejm.org/doi/full/10.1056/NEJMoa1214747 , I hope that this it's useful to you.

Héctor.