The vertebrate eye lens is an organ that is already present in the embryo in early stages of development. One remarkable feature of the lens is that is consists of cell layers, like the layers of an onion. Growth takes place at the outside; the inner part of the lens is therefore just as old as its carrier is! This also means that old cells will not be replaced by new ones contrary to what happens in other parts of the body. This imposes high demands in terms of stability to the constituing parts of these cells. Moreover, it has to be transparant for light - something that quite convenient for a lens...

The alpha-crystallin protein is one of the most important consituent of the eye lens. This protein fullfills a key role in maintaining stability and structure of the lens. Its precise working is not completely known, but we know that the protein is a member of the widely occurring small heat shock protein (HSP20) family. The 3-dimensional structure of a distant relative of the protein has be elucidated some time ago. Since alpha-crystallin plays such an important role (see 1), it evolves at a very slow rate.

1. Calculation of the alpha-crystalline tree

   Use the protein sequences that you can find here. Select them with your mouse ("cut and paste "), and put them in the CLUSTALW program to align them, and subsequently calculate the Neighbor-Joining tree. Alternatively, you can use the "upload" function in the webpage to send the file to the server.

2. A tree with an "error"?

   Now use the sequences you find in this file, and calculate the tree with CLUSTALW.

   What is strange about this tree? Can you explain this? Maybe the final exercise will make clear what we are looking at...

3. The complete picture

   Finally we put all the alpha crystallins in the final sequence file in CLUSTAL and calculate our tree.

   If you didn't get it what was happening in exercise two, then this should clarify the picture that we got in the previous exercise.