Is the phrase "transmembrane segment" equivalent to the transmembrane domain of a protein?

Question

I am reading the Handbook of Neurochemistry and Molecular Neurobiology and I am learning about the cell adhesion molecule NCAM2 and I have come across the following:

The overall structure of NCAM2 is similar to that of NCAM. Thus, NCAM2 consists of five Ig‐modules (supposedly of the C2‐type) followed by two Fn3‐modules. The protein exists in a GPI‐anchored isoform and in a transmembrane isoform that contains a 20–25 amino acid transmembrane segment followed by a 106–119‐amino acid cytoplasmic tail (Alenius and Bohm, 1997; Yoshihara et al., 1997).

I know that NCAM2 is located at the cell membrane and has an extracellular, transmembrane and intracellular domain. However, for the above statement: The protein exists in a GPI‐anchored isoform and in a transmembrane isoform that contains a 20–25 amino acid transmembrane segment followed by a 106–119‐amino acid cytoplasmic tail, is the term 'transmembrane segment' equivalent to the transmembrane domain? Further is the term 'cytoplasmic tail' equivalent to the intracellular domain? Any insights are appreciated.

Answer 1

Yes

I'll admit defeat in trying to find the section in the text you linked - if there's any one thing Google can't do, it's search. But questions like this are better put to Uniprot, which gives quite a bit of information. Following that link, you'll see they identify 21 AA of transmembrane topology from 698 to 718, with a cytoplasmic domain afterward. So the cytoplasmic tail is the intracellular domain.

In a sense there is a second transmembrane domain - the signal peptide from 1-19 - but because it is cleaved little attention is paid to it. The extracellular domain starts at 20, with an N terminus outside the cell.

To quote Wikipedia, a transmembrane domain "usually denotes a single alpha helix of a protein". You should probably listen to them on such an issue because nobody is more fixated on semantics (it's a sort of vote). Philosophically, you can picture that multiple transmembrane segments could operate as a functional unit - but each of these segments would be separated by some region of extracellular or intracellular sequence from the next. A "domain" in a protein is a self-contained unit in the primary sequence, not just a series of helices that turn out to be near each other, so that wouldn't work. And in fact if you look at a GPCR paper they speak in standard terms of seven transmembrane domains even while analyzing how they function together.