In bacteria amyloid fibrils are involved in biofilm formation and cell/cell cell/substrate adhesion. Amyloid comprised of curli proteins in E.coli and Salmonella are a well-described example (see figure). Functional amyloid is also produced by Saccharomyces and Candida
The oval-shaped things are bacteria; the mesh-like stuff in between them is curli amyloid.
A bacterial biofilm. Credit: Janice Haney Carr in Science Daily
The presentation in Rome I mentioned at the outset of this post discussed yet another functional role for amyloid in humans. Peptide hormones in the pituitary gland are actually stored as amyloid within secretory granules. It is postulated that this structure permits on the one hand, enduring stability in a highly concentrated state, and on the other, a means of controlled release of the hormones in question. Click here to read a publication from the authors of that presentation.
There are illnesses characterized by abnormal amyloid or amyloid-like protein aggregates inside cells ("inclusion bodies"). An example would be Lewy Bodies in the neurons of patients with Parkinson's Disease. This type of protein accumulation is not functional amyloid, as it is generally damaging to the cell.
One can hope that understanding how cells handle functional intracellular amyloid will eventually provide insights into the pathophysiology of these disease states and perhaps lead to effective treatments.