ULTRASTRUCTURE OF THE YEAST ACTIN CYTOSKELETON AND ITS ASSOCIATION WITH THE PLASMA-MEMBRANE

We characterized the yeast actin cytoskeleton at the ultrastructural l evel using immunoelectron microscopy. Anti-actin antibodies primarily labeled dense, patchlike cortical structures and cytoplasmic cables. T his localization recapitulates results obtained with immunofluorescenc e light microscopy, but at much higher resolution. Immuno-EM double-la beling experiments were conducted with antibodies to actin together wi th antibodies to the actin binding proteins Abp1p and cofilin. As expe cted from immunofluorescence experiments, Abp1p, cofilin, and actin co localized in immuno-EM to the dense patchlike structures but not to th e cables. In this way, we can unambiguously identify the patches as th e cortical actin cytoskeleton. The cortical actin patches were observe d to be associated with the cell surface via an invagination of plasma membrane. This novel cortical cytoskeleton-plasma membrane interface appears to consist of a fingerlike invagination of plasma membrane aro und which actin filaments and actin binding proteins are organized. We propose a possible role for this unique cortical structure in wall gr owth and osmotic regulation.