ER membrane aquaporins in plants

Plant has a large aquaporin family with more than 30 members which are divided into four subfamilies: plasma membrane intrinsic protein (PIP), tonoplast intrinsic protein (TIP), nodulin 26-like intrinsic proteins (NIP), and small and basic intrinsic proteins (SIP). Their primary structure, transport substrate, functional regulation, gene expression profile, protein amount, and intracellular localization are diversified. The SIP members have short N-terminal tails. Most aquaporins have two sets of common Asn-Pro-Ala (NPA) motif; however, the first motif of SIP1;1, SIP1;2, and SIP2;1 is changed to NPT, NPC, and NPL, respectively. SIP1;1 and SIP1;2, but not SIP2;1, have water transport activity. A recent study revealed that all three members of SIP are localized to the ER membrane and expressed in a cell specific manner in Arabidopsis thaliana. An overview is given on the main features of the SIP members in terms of their primary structure, ER membrane retention, homologues in mammals, and physiological function.