Fine (2-5-nm) filaments: new types of cytoskeletal structures.

Over the past 30 years filaments 2-5 nm in diameter have been found in a number of different types of eukaryotic cells. As a group, these fine filaments lack the similarity of composition and function that characterize the three major classes of cytoskeletal elements--microfilaments, microtubules, and intermediate filaments. Six different proteins that form fine filaments have been identified; proposed functions for these fibers range from cell motility to cytoarchitecture. Recent studies, however, have revealed filaments with similar compositions and/or functions in otherwise different cells, suggesting that the fine filaments may eventually fit into a limited number of subgroups.