phasins

Phasins are a family of small, granule-associated proteins found in many bacteria that store carbon as polyhydroxyalkanoates (PHA) in cytoplasmic inclusions known as PHA granules. They coat the hydrophobic surface of the granule and play a key role in determining granule surface properties and dynamics within the cell.

Phasins are encoded by phaP and related genes within PHA biosynthesis clusters. They can exist as multiple isoforms with differing affinities for PHA, and may be expressed in response to the cell’s carbon and nutrient status. Their binding to the granule surface is often mediated by amphipathic helices and other hydrophobic interactions, enabling stable association with the inclusion.

Functionally, phasins regulate the number, size, and intracellular localization of PHA granules. By influencing granule stability and surface accessibility, they can affect the turnover of PHA by depolymerases and the efficiency of synthesis by the PHA synthase complex. The presence and relative abundance of different phasin isoforms can shape granule morphology, particularly in species that harbor more than one phasin protein.

Diversity among phasins is common across PHA-producing bacteria. Some species possess several phasins (for example, phaP1 and phaP2), with distinct expression patterns and binding properties that together influence granule formation. The proteins often form oligomeric assemblies on the granule surface, creating a dynamic interface between the polymer and the cellular milieu.

Applications include using phasins as fusion partners to attach fluorescent or other reporter tags to PHA granules, enabling visualization of storage dynamics. Manipulating phasin expression is also explored to tune PHA yield and granule characteristics in engineered microbes.