The narrowest fibres of the cytoskeleton are the microfilaments, which are built from:

1. Each class of cytoskeletal fibre is built from its own protein. 2. Of the three types of protein fibre in the cytoskeleton, microfilaments are the narrowest. 3. They are made of two intertwined strands of a globular protein called actin. 4. Tubulin builds microtubules, and keratin is one of the intermediate filament proteins. _Source: OpenStax Biology for AP(R) Courses (CC BY 4.0), Ch 4 'Cell Structure', sections 4.1-4.6_