phasin

Phasin is a family of small, typically amphipathic proteins that coat intracellular polyhydroxyalkanoate (PHA) granules in many bacteria and some archaea. Phasins are encoded by phaP genes and are often located near the phaCAB operon that encodes enzymes for PHA biosynthesis. They play a central role in the maturation, stabilization, and regulation of PHA inclusions.

Phasins bind to the hydrophobic surface of PHA granules through amphipathic helices, helping to stabilize the granule core and influence granule number and size within the cell. They can modulate access of depolymerases to the polymer, and in some species interact with PHA synthase or other enzymes involved in PHA metabolism, thereby affecting polymer accumulation and turnover. In certain bacteria, different phasin proteins can have distinct roles or be differentially expressed under varying carbon conditions.

Structure and diversity among phasins are broad. Most are small proteins, typically in the range of 10–25 kDa, and show sequence diversity that supports their classification into several families. They often possess targeting motifs that direct them to PHA granules and may form oligomers that contribute to their functional properties on the granule surface.

Genetic organization and regulation vary by species. Many bacteria harbor multiple phasin genes, and expression levels often correlate with the cellular PHA content. Deletion or overexpression of phasin genes can alter granule morphology, number, and overall PHA yield, highlighting their role in granule dynamics.

Significance and applications: Phasins are studied for their potential to optimize bioplastic production, as manipulating phasin expression can tailor PHA content and granule characteristics. They remain a key focus in research on the cellular biology of PHA storage and its biotechnological exploitation.