Proteins (and amyloids) still have some tricks up their sleeves

We all know that amyloids are made of beta sheets stacked on top of each other. Not all of them, says Staph Aureus according to PNAS e2014442118 ’21. In fact one protein they produce (Phenol Soluble Modulin alpha 3 (PSMα3)– PSMalpha3 ) which is toxic to human immune cells forms amyloid made of alpha helices.  PSMalpha3 forms cross-α amyloid fibrils that are composed entirely of amphipathic α-helices. The helices stack perpendicular to the fibril axis into mated “sheets”

However other members of the family namely PSMα1 and PSMα4 adopt the classic amyloid ultrastable cross-β architecture and are likely to serve as a scaffold rendering the biofilm a more resistant barrier.

It gets worse.

Consider an antimicrobial peptide (AMP) called uperin 3.5, secreted on the skin of a frog which also forms amyloid fibrils made of alpha helices.  The amyloid is  essential for uperin 3.5’s  toxic activity against the Gram-positive bacterium Micrococcus luteus.

It gets even worse.  

When secreted onto the frog skin uperin 3.5. has a disordered structure. Uperin 3.5 requires bacterial membranes to form the toxic amyloid made of alpha helices.   When no membranes are around, uperin 3.5. still forms amyloid, but this time the amyloid is of the classic beta sheet.  So one protein can form two types of amyloid.  Go figure

Uperin 3.5 is a classic example of a chameleon protein. 

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