I have a vague idea of what you're getting at, what you want to do is keyframe the position of your text and either keframe the size or else put it behind a mask layer.
When I add a mask to the text layer it moves along with the text. I can't keyframe the mask independently.
if you wan't separate control for the mask and the element you're masking, an alpha matte is a way to do it.
1. create your mask on a solid layer or create a shape layer
2. set the text layer below the masked solid or shape layer
3. under the transfer controls column, set the text to "Alpha matte"
I can't keyframe the mask independently.
By definition the mask's keyframes are "independent" in that their property stream works separate from all others. Of course this doesn't help you. what you actually will want is to use a linear wipe effect on your pre-composed text or a static solid as a matte for your text to define an edge where the text is cut off and disappears into the line. I would suggest you look up the big words in the help or a web search. This is nothing complicated, but it's rather obvious that you probably never used AE before.