Cloning and expression of a major rat lens membrane protein, MP20.

The lens contains a major membrane protein with an apparent molecular weight of 18 kDa and which has been referred to as MP18 [Louis et al. (1989). J. Biol. Chem. 264, 19967-73]. We have cloned a rat MP20 cDNA that appears to be identical to this previously described protein based on sequence homology. The predicted protein sequence has characteristics typical of an integral membrane protein and a molecular mass of 19637. The transcripts for MP20 appear to be lens specific as indicated by RNA analysis; Southern blot analysis indicates that MP20 is coded for by a single gene with possibly a single intron in its coding sequence. The expression of transcripts for several lens membrane proteins (MP20, MP26 and alpha 3-gap junctions) was compared in 1.5 month bovine fetal and 4-6 month post-natal calf lenses. The transcript levels for these proteins were more abundant in fetal lenses than in calf lenses indicating a possible developmental regulation of the transcripts for these three lens-specific membrane proteins. The MP20 cDNA was expressed in a heterologous mammalian cell culture system in which the majority of the protein was integrated into membranous structures localized near the nucleus; there was little incorporation of MP20 into the cell plasma membrane as detected by immunofluorescence. This system should prove useful for both the isolation and purification of MP20, and will enable study of its properties under defined conditions.