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It all depends on how you want the line to behave. You can start by drawing an ellipse shape layer with stroke only while holding down the Alt/Option and shift key so you bet a path instead of a parametric shape, then view the comp at 1600% magnification factor and use the pen tool to add a point right next to one of the 4 that are formed when you created the circle by using the pen tool (g). Then hold down the Ctrl/Cmnd key and drag a selection around the two points that are close together, right click, go to mask properties and select open path.
Now set a keyframe for path.
Now start a new straight line using the pen tool. Your circle has 5 points so create five points the same distance apart using the pen tool. Set a keyframe for this second path and then cut it, then delete the path
Now move down the timeline a few frames, select the circle path and paste. You have just animated a circle turning into a line. The points on the path are going to travel to their new positions in a straight line so you may get some interesting results.
If you have no idea what I'm talking about then type edit shape layers in the search help field at the top right corner of AE and read up on how to draw and animate shape layers.
The other way is to create a circle and a line in Illustrator then use the blend tool to create a transition and then . . . Well something like in this tutorial I did a while ago.
Another way would be to set up a circle shape layer, like you did in the first example, then click on the original point with the pen tool and add another point to the side of the circle that is long enough to be the straightened line. Then add Trim Paths and animate the start and end point so that it looks like the circle is unwinding into a straight line.
Or . . . Well there are about a dozen other ways I can think of to do this. Unfortunately there is no button you can push to make it happen automatically. It would help if you gave a more detailed description of what you want the animation to look like.