Question

Imagine a transmembrane protein consisting of a single polypeptide chain. The protein has alpha helices that pass through the membrane seven times. Between each segment that passes through the membrane are loops that extend into the cytoplasm or into the extracellular fluid. The C-terminus of the protein extends into the cytoplasm and the N-terminus extends into the extracellular fluid.​ What is the minimum number of polar stretches* in the primary structure of this protein?

*polar stretches are contiguous regions of the polypeptide that are rich in polar amino acids.

	A.	Two
	B.	Four
	C.	Six
	D.	Seven
	E.	Eight

Answer 1

Answer: E: Eight

There will be at least 8 polar stretches in the primary structure of the protein as indicated in the image attached. (Polar stretches are numbered in red). In a transmembrane(TM) protein, the TM domains are composed of mostly hydrophobic residues that suit the hydrophobic environment of the membrane core. On the flip side, the residues that connect the TM domains and hang out in the cytoplasm or in the extracellular fluid are mostly polar. Please note that it is also possible to have polar stretches inside the TM domain, but the chances are less. Refer to the image for better understanding.