This domain inhibits pectin methylesterases (PMEs) and invertases through formation of a non-covalent 1:1 complex. It has been implicated in the regulation of fruit development, carbohydrate metabolism and cell wall extension. It may also be involved in inhibiting microbial pathogen PMEs. It has been observed that it is often expressed as a large inactive preprotein. It is also found at the N-termini of PMEs predicted from DNA sequences (personal obs:C Yeats), suggesting that both PMEs and their inhibitor are expressed as a single polyprotein and subsequently processed. It has two disulphide bridges and is mainly alpha-helical.