Which two amino acids is NLS rich in?

The nuclear localization signal (NLS) is a crucial signal for the transport of proteins into the nucleus, and it is characteristically rich in basic amino acids. The two amino acids that are most commonly found in high concentrations within NLS are lysine and arginine.

Lysine and arginine both possess positive charges at physiological pH, which enables them to interact with the negatively charged nucleic acids (DNA and RNA) within the nucleus. This interaction is essential for the recognition and transport of proteins through the nuclear pore complex, facilitating the nuclear import process.

The other options involve amino acids that do not share this basic, positively charged characteristic. For instance, serine and threonine are polar, but they lack the essential positive charge needed for nuclear localization signals. Similarly, valine, leucine, alanine, and glycine are more hydrophobic or neutral and do not contribute to the NLS's required properties for nuclear import. Thus, the correct choice highlights the specific structural and functional roles lysine and arginine play in the NLS.