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In addition to roles as catalysts and signalling molecules, proteins also perform the more modest role as structural or mechanical proteins. The fibrous extracellular matrix proteins have already been mentioned, but inside the cell there are many more. The histones are small globular proteins around which chromosomal DNA wraps itself for efficient packaging during nuclear division. DNA packaging probably also has a role in gene expression. The uptake of molecules into cells by endocytosis involves the invagination of the cell membrane to form vesicles. Vesicle formation throughout the cell is aided by proteins like clathrin which associate in a polyhedral cage-like structure and promote the curvature of the membrane.
Force generating cellular machinery also exists: muscle is a linear motor which utilises conformational change and a ratchet-like mechanism. Intracellular transport along cytoskeletal microtubules uses a similar mechanism. Breathtakingly elegant rotary mechanisms also exist. The bacterial flagellum is a large multi-protein assembly which can rotate at over 100 revolutions per second. The recently solved structure of the F1 subunit of ATP synthase[Abrahams et al., 1994,Noji et al., 1997] showed that rotary mechanisms are possible in much smaller protein assemblies, in this case involving only 7 polypeptide subunits.