Plasma membrane intrinsic proteins from maize cluster in two sequence subgroups with differential aquaporin activity

The transport of water through membranes is regulated in part by aquaporins or water channel proteins. These proteins are members of the larger family of major intrinsic proteins (MIPs). Plant aquaporins are categorized as ei ther tonoplast intrinsic proteins (TIPs) or plasma membrane intrinsic prote ins (PIPs). Sequence analysis shows that PIPs form several subclasses. We r eport on the characterization of three maize (Zea mays) PIPs belonging to t he PIP1 and PIP2 subfamilies (ZmPIP1a, ZmPIP1b, and ZmPIP2a). The ZmPIP2a c lone has normal aquaporin activity in Xenopus laevis oocytes. ZmPIP1a and Z mPIP1b have no activity, and a review of the literature shows that most PIP 1 proteins identified in other plants have no or very low activity in oocyt es. Arabidopsis PIP1 proteins are the only exception. Control experiments s how that this lack of activity of maize PIP1 proteins is not caused by thei r failure to arrive at the plasma membrane of the oocytes. ZmPIP1b also doe s not appear to facilitate the transport of any of the small solutes tried (glycerol, choline, ethanol, urea, and amino acids). These results are disc ussed in relationship to the function and regulation of the PIP family of a quaporins.