Developmental changes in membrane protein expression by chick lens cells in vivo and in vitro and the detection of main intrinsic polypeptide (MIP).

We have compared the long-term developmental changes in water-insoluble protein expression by chick lens cells in vitro and in vivo. Crude membrane fractions were prepared by alkali treatment of the urea-insoluble protein fraction, and the proteins analysed by sodium dodecyl sulphate-polyacrylamide (SDS-PAGE) gel electrophoresis. The major component present in the urea-insoluble fraction of chick lens fibres, a 25000 MW polypeptide (MIP-25K) was more abundant in adult (8 weeks) than day-old post-batch chick lens fibre masses. MIP-25K was detected in differentiated but not predifferentiated lens cell cultures, and indirect immunofluorescence using anti-bovine MIP antiserum indicated that MIP-25K was localized in the lentoid bodies. Our findings indicate that the urea-insoluble protein profiles of long-term well-differentiated chick lens cell cultures are qualitatively very similar to the profiles of the lens fibres. The data also confirm that the expression of MIP-25K, rather than the expression of water-soluble crystallin protein, is a marker for lens cell differentiation, and confirm earlier reports, which have been disputed, that δ-crystallin (but not α- or β-crystallin) is specifically associated with chick lens fibre membranes.