MP20, the second most abundant lens membrane protein and member of the tetraspanin superfamily, joins the list of ligands of galectin-3

MP20 and galectin-3 co-localized in selected areas of the lens fiber cell plasma membrane. Individually, these proteins purified with apparent molecular masses of 60 kDa and 22 kDa, respectively. A 104 kDa complex was formed in vitro upon mixing the purified proteins. A 102 kDa complex of MP20 and galectin-3 could also be isolated from detergent-solubilized native fiber cell membranes. Binding between MP20 and galectin-3 was disrupted by lactose suggesting the lectin site was involved in the interaction.MP20 adds to a growing list of ligands of galectin-3 and appears to be the first representative of the tetraspanin superfamily identified to possess this specificity.Tetraspanins constitute a superfamily of integral membrane proteins, which share a common membrane topology characterized by four membrane-spanning segments and the location of amino- and carboxy-termini at the cytoplasmic surface [1]. Most members further have a consensus N-linked glycosylation site in one of the extracellular loops [2]. Tetraspanins have been implicated in a variety of cellular processes including activation, proliferation, differentiation, migration, adhesion, and apoptosis [3]. Tetraspanins form contacts with other cells or the extracellular matrix by binding to other tetraspanins, to adhesion receptors such as integrins, and to extracellular proteins [4-6].The PMP22/EMP/MP20 gene family is a subfamily of the tetraspanins [7]. PMP22, also known as CD25, has been associated with fibroblast apoptosis [8]. It is also expressed in myelinating Schwann cells where it plays a role in differentiation [9-11]. When absent or dysfunctional, it causes peripheral neuropathies that result in progressive distal muscle weakness [12]. This report is concerned with MP20 [13,14]. Previously also referred to as MP17 and MP18, it is the second most abundant integral membrane protein of lens fiber cells, which appears to be distributed uniformly in the plasma membrane but also occurs in distinct membrane