Organization of an actin filament-membrane complex. Filament polarity and membrane attachment in the microvilli of intestinal epithelial cells.

TLDR

Actin filament–membrane association is a key determinant of nonmuscle cell motility, and in intestinal microvilli this association is visualized through actin paracrystals in the filament bundles. The study investigates the highly ordered actin filament–membrane complex of the intestinal brush border and proposes a functional organization model. The authors present a model describing how actin and myosin are arranged within the brush border. All microvillar actin filaments share the same polarity with S1 arrowheads pointing away from the apical tip, are linked to the plasma membrane along their lengths by periodic 33‑nm cross bridges (best seen with high Mg²⁺), and myosin‑like filaments are also present in the terminal web.

Abstract

The association of actin filaments with membranes is now recognized as an important parameter in the motility of nonmuscle cells. We have investigated the organization of one of the most extensive and highly ordered actin filament-membrane complexes in nature, the brush border of intestinal epithelial cells. Through the analysis of isolated, demembranated brush borders decorated with the myosin subfragment, S1, we have determined that all the microvillar actin filaments have the same polarity. The S1 arrowhead complexes point away from the site of attachment of actin filaments at the apical tip of the microvillar membrane. In addition to the end-on attachment of actin filaments at the tip of the microvillus, these filaments are also connected to the plasma membrane all along their lengths by periodic (33 nm) cross bridges. These bridges were best observed in isolated brush borders incubated in high concentrations of Mg++. Their visibility is attributed to the induction of actin paracrystals in the filament bundles of the microvilli. Finally, we present evidence for the presence of myosinlike filaments in the terminal web region of the brush border. A model for the functional organization of actin and myosin in the brush border is presented.