**3. CGP characteristics**

In 1971, Robert Simon isolated CGP granules for the first time by using differential centrifugation. Along with this study, CGP has shown its special and unique solubility behavior [26]. CGP is insoluble at physiological ionic strength and at neutral pH, but soluble in solutions which are acidic, basic or highly ionic. In non-ionic detergents such as Triton X-100, CGP is insoluble; however, in ionic detergents like SDS, it is soluble [6]. Present-day CGP extraction methods are based on its solubility at low pH and insolubility at neutral pH [27].

The chemical structure of CGP was proposed in 1976 by Simon and Weathers [2]. According to this model, CGP has a polymer backbone consisting of α-linked aspartic acid residues. The α-amino group of arginine is linked via isopeptide bonds to the β-carboxylic group of every aspartyl moiety. Because every aspartate residue is linked to an arginine residue, CGP contains equimolar amounts of aspartate and arginine [2]. This structure has been confirmed via enzymatic degradation studies. CGP-degrading enzymes (see below) release β-Asp-Arg dipeptides [28]. CD spectroscopy data suggest that the acid-soluble and neutral insoluble forms of CGP have similar conformations. Both forms contain substantial fractions of β-pleated sheet structure [29].

Cyanobacterial CGP has a molecular weight and polydispersity ranging from 25 to 100 kDa [26]. In contrast, the native CGP producer *Acinetobacter* sp. ADP1 synthesizes CGP with a lower molecular weight ranging from 21 to 28 kDa [30]. Recombinant bacteria or genetically engineered yeast harboring heterologous expression of cyanobacterial CGP synthesis genes also show a lower molecular weight of 25–45 kDa [27, 31]. Transgenic plant-produced CGP also shows a reduced polydispersity between 20 and 35 kDa [32]. A possible explanation would be that cyanophycin synthesis in the native cyanobacterial background involves additional factors contributing the polymer length. These additional factors should also be absent in *Acinetobacter* sp. ADP1.

Native CGP is exclusively composed of aspartate and arginine. By contrast, in CGP isolated from recombinant *E. coli* expressing cyanophycin synthetase (see below) from *Synechocystis* sp. PCC 6803, besides aspartate and arginine, lysine has been found [33]. The amount of incorporated lysine in CGP influences its solubility behavior. Recombinant CGP with a high lysine amount (higher than 31 mol%) is soluble at neutral pH [34].
