A phasin, it attaches to the polyhydroxybutyrate (PHB) granule surface regulating the number and size of PHB granules within a cell. It probably also acts as a regulator affecting the biosynthetic activity of PHB synthase in vivo. Biopolymer metabolism; poly-(R)-3-hydroxybutanoate biosynthesis. Homotetramer. Distributed in cytoplasm during photoautotrophic growth, insoluble and associated with polyhydroxybutyrate (PHB) granules under nitrogen starvation (PubMed:25911471, PubMed:9683655). Constitutively expressed, increasing levels as growth progresses (at protein level). Probably has 2 alpha-helices (resides 3-26 and 39-83); both helices alone target to PHB granules, although helix 2 may be the main anchor. Both helices are required for oligomerization. During N(2) starvation fewer PHB granules accumulate per cell, while their diameter is increased by about 25% after 6 days of nitrogen starvation. Decreased rate of PHB synthesis, however overall accumulation of PHB is not significantly different. Poly(3-hydroxyalkanoic acids) (PHA), of which PHB is among the most common compounds, are prokaryotic intracellular storage compounds with potential uses as a renewable, biodegradable thermoplastic. Cyanobacterial PHB synthesis is particularly attractive as cyanobacteria use CO(2) as the carbon source. Nitrogen-free medium induces chlorosis in Synechocystis, leading to the degradation of the photosynthetic apparatus and concomitant accumulation of cytoplasmic polyhydroxyalkanoic acid (PHA) granules which in this cyanobacterium are composed of PHB.