Final answer:
A tag is known to be at the N-terminus of a protein based on the conventional representation of proteins from N- to C-terminus, the use of specific reagents that identify free amino groups at the N-terminus, and signal sequences that are removed post-translationally, which indicate the N-terminal location.
Step-by-step explanation:
To determine whether a tag is at the N-terminus of a protein, we can look at several factors. Firstly, proteins and polypeptides are conventionally written and synthesized from N-terminus to C-terminus. The N-terminal amino acid is the one with a free amino group (-NH2), which is indicated when written as the leftmost amino acid on a horizontally displayed peptide. Reagents such as Sanger's reagent, Edmann's reagent, and Dansyl chloride are used to identify the N-terminal amino acid, indicating its presence on that end of the peptide. Moreover, certain signal sequences, consisting of 10-30 amino acids at the N-terminal end, are known to direct proteins to their target locations within the cell, and these sequences are typically removed post-translationally, signifying the N-terminal location. The location of the N-terminal end can also influence protein stability, as explored in the N-end rule, which shows certain amino acids at the N-terminus can stabilize or destabilize the protein, further affecting its function and lifespan.
Considering the physiological role and insertion into cellular structures, such as with transmembrane proteins, the orientation and thus the N-terminus can be deduced. These proteins can cross the membrane multiple times, and the N-terminal end can be positioned either inside or outside of the cell depending on how the protein inserts into the membrane during biosynthesis. For instance, in some plasma membrane proteins, if the N-terminal end is exposed to the outside, this is an indication of the N-terminus. All these factors together provide a comprehensive method of confirming the presence of a tag at the N-terminus of a protein.