Look at the fastA file: sequences are always listed N-term to C-term. In the same way, the most N-terminal of a protein chain comes first in the coordinate file, followed by the other residues in order, until the C-terminal residue terminates the chain. C-terminus means the end of the chain that holds the backbone COO- group (carboxylate, whence "C"). The C-terminus does not really have a length, it is the breakout-point of the chain. If a paper says that 6 C-terminal residues were disordered and therefore invisible in the structure, it means that the last six residues could not be seen.

In order to define the N and C terminal you may install VMD. Follow the link: http://www.ks.uiuc.edu/Research/vmd/current/ig/ig.html

Annemarie Honegger Thank you so much for your helpful answer. I will take this opportunity to ask a more specific question; for example, when someone is talking about the C-terminal tail of a protein, is there a way to determine how many residues are there in the "tail"?

I hope I can learn a bit more from here!

How many residues are in the "tail" is dependent on the specific protein/sequence. Such a "tail" may consist of specific sequences added to the construct to facilitate purification and/or detection, e.g. his-tag, myc-tag, to control subcellular localisation, e.g. ER-retention signal, or of intrinsic parts of the sequence or the protein that remain unfolded and flexible in the native structure