For decades, scientists have known that proteins searching for genetic sequences are able to locate them at rates much faster than expected. They found that rather than moving around the entire three-dimensional space inside a cell, they moved in one-dimension, along DNA molecules. The Harvard group showed, in 2006, that the proteins slide back and forth in direct contact with the DNA as part of the search for specific sequences.
Until now, however, the exact nature of the path these molecules take along the DNA has not been known. Competing biological models assert that the proteins either move in a straight line parallel to the DNA axis or trace more complex helical paths, following a strand or groove of DNA around that axis.
Depending on how a protein moves along a DNA axis — either in a linear or helical pattern —it will encounter different degrees of resistance, as shown in the earlier paper. If protein motion is linear, its speed will decrease proportionately as its radius increases. If a protein exhibits helical motion, it will experience additional friction and its speed will decrease much faster as its radius increases.
Using a human DNA repair protein as a test for the protein rotation model, Paul Blainey, now at Stanford University, found the latter case to be true. When he increased the size of the protein, the rate of motion decreased much more rapidly than it would have for a simple linear motion.