Structural Arrangement of Lens Fiber Cell Plasma Membrane Protein MP20

Phosphorylation and Glycosylation of Bovine Lens MP20

“…The proposed structure of the MP20 domains that fits with the predicted extracellular locations of the glycosylated residues and the intracellular location of phosphorylated residues (Fig. 5) is similar to those proposed by Kumar et al 12 and Taylor et al 7 In contrast, the glycosylated tryptophan residues would not be present in extracellular domains in models proposed by Arneson and Louis 11 and Chen et al 13 In work by Gray et al, 3 MP20 localization changed from the cytoplasm in the outer cortex to the membranes of the fiber cells on differentiation. Furthermore, work by Tenbroek et al 2 showed that between 0.5 and 2.0 mm into the lens, MP20 localization changed from patterns dispersed on the broad sides of fiber cells to the narrow interdigitated sides.…”

“…Although the topology of MP20 within the fiber cell membrane is not known, it has been shown to form hexamers within the membrane by cross-linking studies. 11 Oligomeric formation is further supported by the observed dominant negative behavior of an MP20 mutant protein in the MP19 T03 transgenic mouse. 6 There is contradictory evidence of the topological structure of MP20.…”

“…38 Binding of galectin-3 to the sites of glycosylation in MP20 is further supported by the observation that antibodies to residues 26-44 and 50-65 had increased binding to MP20 after a short exposure to trypsin, possibly due to the removal of a protein that blocked access to these regions. 11 From the data presented in this study, several hypotheses may be made about the structure and function of MP20 in the lens. The proposed structure of the MP20 domains that fits with the predicted extracellular locations of the glycosylated residues and the intracellular location of phosphorylated residues (Fig.…”

“…7 Kumar et al 12 proposed an MP20 structure that is most similar to the proposed PMP22/EMP structure with the N terminus located in the first membrane-spanning region and residues 25-63 in the first extracellular domain. In contrast, Arneson and Louis 11 proposed that a large part of the N-terminal region of MP20 (residues 1-65) is located intracellularly. Last, Chen et al 13 proposed that MP20 has five membrane-spanning regions with residues 1-21 in the first membrane-spanning region and residues 22-64 in a large intracellular loop.…”

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

“…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 junctional domains [15-17]. Mutations in MP20 severely disrupt the normally crystalline fiber cell arrangement in the lens and cause cataractogenesis [18,19].…”