Scanning tunneling microscopy of cytoskeletal proteins: Microtubules and intermediate filaments

Direct STM observation of native biomolecules has proven feasible. In this study we have used STM to image two filamentous protein components of the intracellular cytoskeleton: microtubules (MT) and intermediate filaments (IF). MT are 25 nm diameter cylinders comprised of 13 “protofilaments” which are linear chains of 8 nm X 4 nm X 4 nm “dimer” subunits. More variable than MT, IF are 10 nm diameter coils comprised of from 4 to 8 “subfilaments” which are chains of “tetramer” subunits. MT were isolated from porcine brain and IF from cell culture by standard techniques. Preparation/stabilization factors included magnesium buffer, 0.8 molar glycerol, and 0.1% glutaraldehyde. Samples were scanned on graphite in a Nanoscope I or II STM (Digital Instruments, Santa Barbara, California). STM images of MT demonstrated flattened 25 nm diameter structures composed of 4 nm wide protofilaments. Processed inverted images showed rows of 8 nm X 4 nm subunits. STM images of IF showed flattened, parallel 10 nm filaments comprised of coiled chains of 6–12 nm tetramer subunits. Thus STM demonstrated internal structure of two different classes of cytoskeletal proteins. STM and related techniques have an important future in biomolecular studies.