Mercury-induced conformational changes and identification of conserved surface loops in plasma membrane aquaporins from higher plants. Topology of PMIP31 from Beta vulgaris L.

Aquaporins are integral membrane proteins occurring in mammals, plants, and microorganisms, which serve as channels that permit the bidirectional passage of water through cellular membranes. Higher plants contain abundant levels of aquaporins in both the tonoplast and plasma membrane. Aquaporins contain six transmembrane segments with three surface loops located at the apoplastic face of the membrane and two loops at the cytosolic side. In this study, we probed the topology of plasma membrane aquaporins to determine the effects of divalent cations on aquaporin conformation, and to identify structural features that distinguish plasma membrane intrinsic proteins from tonoplast intrinsic proteins. Plasma membrane vesicles from storage tissue of Beta vulgaris L. were subjected to limited proteolysis, and proteolytic fragmentation patterns were detected using affinity-purified antibodies recognizing aquaporins of 31-kDa. In its native membrane-associated state, the 31-aquaporin band, PMIP31, was refractory to proteolysis by trypsin. However, mercuric compounds specifically induced a conformational change resulting in the exposure of a proteolytic cleavage site and formation of a unique 22-kDa proteolytic fragment (p22). N-terminal sequence analysis of p22 established its identity as an aquaporin-derived fragment. Topological studies using sealed right-side-out plasma membrane vesicles established that the proteolytic cleavage site is located at surface loop C, the second apoplastic loop, immediately preceding the sequence Gly-Gly-Gly-Ala-Asn. The Gly-Gly-Gly-Ala-Asn-X-X-X-X-Gly-Tyr motif of loop C and a 14 amino acid motif in apoplastic loop E, Thr-Gly-Ile/Thr-Asn-Pro-Ala-Arg-Ser-Leu/Phe-Gly-Ala-Ala-Ile/Val-Ile/ Val-Phe/Tyr-Asn are completely conserved in all known higher plant aquaporins of plasma membrane origin and are not present in any of the known tonoplast intrinsic proteins. These results demonstrate that the two highly conserved plasma membrane intrinsic protein surface loops are structural features that clearly distinguish plasma membrane from tonoplast aquaporins.