In order to have its n-terminus extending into the cytoplasm, the cftr protein must have contained a(n) as the first signal peptide encountered in the primary sequence.

In order to have its n-terminus extending into the cytoplasm, the cftr protein must have contained a NBD as the first signal peptide encountered in the primary sequence.

Explanation:

Cystic Fibrosis Trans-membrane conductance regulator (CFTR) exhibits the two domain swap structure and is a protein found in the pancreas, intestinal and respiratory epithelia. The N-terminal signal functions as a first signal peptide.

CFTRs helps in up taking nutrients in prokaryotes and export variety of substrates in the eukaryotes. This NBD nucleotide binding domain acts as the first signal peptide to be encountered in the primary sequence.

i think the answer is a.keratin is the pigment responsible for skin color.