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Annexins constitute an evolutionary conserved multigene protein superfamily characterized by their ability to interact with biological membranes in a calcium dependent manner. They are expressed by all living organisms with the exception of certain unicellular organisms. The vertebrate annexin core is composed of four (eight in annexin A6) homologous domains of around 70 amino acids, with the overall shape of a slightly bent ring surrounding a central hydrophilic pore. Calcium- and phospholipid-binding sites are located on the convex side while the N-terminus links domains I and IV on the concave side. The N-terminus region shows great variability in length and amino acid sequence and it greatly influences protein stability and specific functions of annexins. These proteins interact mainly with acidic phospholipids, such as phosphatidylserine, but differences are found regarding their affinity for lipids and calcium requirements for the interaction. Annexins are involved in a wide range of intra- and extracellular biological processes in vitro, most of them directly related with the conserved ability to bind to phospholipid bilayers: membrane trafficking, membrane-cytoskeleton anchorage, ion channel activity and regulation, as well as antiinflammatory and anticoagulant activities. However, the in vivo physiological functions of annexins are just beginning to be established.

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4F2hc (CD98hc, FRP-1, and SLC3A2) is a multifunctional type II membrane glycoprotein involved in amino acid transport (1), cell fusion (2), and ␤1 integrin-dependent adhesion (3). 4F2hc and the homologous rBAT are the heavy subunits of the heteromeric amino acid transporters (HATs), 4 which are linked by a disulfide bridge to the catalytic light subunit (Fig. 1A). One of six light subunits (LAT1, LAT2, y ϩ LAT1, y ϩ LAT2, asc-1, and xCT) heterodimerizes with 4F2hc, thereby rendering a range of transport activities. 4F2hc-associated light subunits are involved in human pathology (yϩLAT1 mutations cause lysinuric protein intolerance, and xCT is the receptor of Kaposi sarcoma-associated herpesvirus (4 -6)). The known role of the heavy subunits is to bring the holotransporter to the plasma membrane. Moreover, 4F2hc is involved in cellular transformation because it is highly expressed in tumor cells; its expression correlates with tumor development, progression, and metastatic potential; and its overexpression leads to cell transformation (7-10). 4F2hc is a mediator of ␤1 integrin signaling (11). Recently, a metabolic activation-related CD147-4F2 complex has been identified on the cell surface that may play a critical role in energy metabolism, probably by coordinating the transport of lactate (via MCT1 and MCT4) and amino acids (via LAT1) (12). Integrin interaction and the CD147-4F2 complex may explain the role of 4F2hc in cellular transformation.The role of the big ectodomain of the heavy subunits of HATs remains largely unknown. The 4F2hc ectodomain (4F2hc-ED) is required for plasma membrane localization of the light subunits LAT2 and y ϩ LAT2 (13). 4F2hc-ED might also modulate ␤1 integrin function and tumorigenicity (10), although interactions with ␤1 integrins involve the transmembrane and the *

Ribotoxins are a family of highly specific fungal ribonucleases that inactivate the ribosomes by hydrolysis of a single phosphodiester bond of the 28 S rRNA. ␣-Sarcin, the best characterized member of this family, is a potent cytotoxin that promotes apoptosis of human tumor cells after internalization via endocytosis. This latter ability is related to its interaction with phospholipid bilayers. These proteins share a common structural core with nontoxic ribonucleases of the RNase T1 family. However, significant structural differences between these two groups of proteins are related to the presence of a long amino-terminal ␤-hairpin in ribotoxins and to the different length of their unstructured loops. The aminoterminal deletion mutant ⌬(7-22) of ␣-sarcin has been produced in Escherichia coli and purified to homogeneity. It retains the same conformation as the wild-type protein as ascertained by complete spectroscopic characterization based on circular dichroism, fluorescence, and NMR techniques. This mutant exhibits ribonuclease activity against naked rRNA and synthetic substrates but lacks the specific ability of the wild-type protein to degrade rRNA in intact ribosomes. The results indicate that ␣-sarcin interacts with the ribosome at two regions, i.e. the well known sarcin-ricin loop of the rRNA and a different region recognized by the ␤-hairpin of the protein. In addition, this latter protein portion is involved in interaction with cell membranes. The mutant displays decreased interaction with lipid vesicles and shows behavior compatible with the absence of one vesicle-interacting region. In agreement with this conclusion, the deletion mutant exhibits a very low cytotoxicity on human rhabdomyosarcoma cells.