Molecular architecture of the lens fiber cell basal membrane complex

Lens fiber cells are transparent, highly elongated, epithelial cells. Becau se of their unusual length these cells represent a novel model system to in vestigate aspects of epithelial cell polarity, In this study, we examined t he fiber cell basal membrane complex (BMC), The BMC anchors fiber cells to the lens capsule and facilitates their migration across the capsule. Confoc al microscopy revealed that bundled actin filaments converge beneath the ce nter of each BMC and insert into the lateral membrane at points enriched in N-cadherin. Two other contractile proteins, caldesmon and myosin, were enr iched in;the BMC, co-localizing with f-actin bundles. The actin/N-cadherin complex formed a hexagonal lattice, cradling the posterior face of the lens , Removal of the capsule caused the tips of the fiber cells to break off, r emaining attached to the stripped capsule, This provided a method for assay ing cell adhesion and purifying BMC components. Fiber cell adhesion require d Mg2+ and/or Ca2+ and was disrupted by incubation with beta 1 integrin ant ibody. BMC proteins were compared with samples from the neighboring lateral membrane, Although some components were common to both samples, others wer e unique to the BMC. Furthermore, some lateral membrane proteins, most nota bly lens major intrinsic protein (MTP), were excluded from the BMC, Western blotting of BMC preparations identified several structural proteins origin ally found in focal adhesions and two kinases, FAK and MLCK, previously und escribed in the lens. These data suggest that the BMC constitutes a distinc t membrane domain in the lens. The structural organization of the BMC sugge sts a role in shaping the posterior lens face and hence the refractive prop erties of the eye.