Which of the following statements about transmembrane proteins is correct? they usually have one or more helical regions spanning the membrane bilayer. they are always inserted with the amino terminus on the lumenal side and the carboxyl terminus in the cytosol. they are always inserted with the carboxyl terminus on the lumenal side and the amino terminus in the cytosol. the signal sequences are always cleaved off. they can cross the membrane only once.

The correct answer is: They usually have one or more helical regions spanning the membrane bilayer

Transmembrane proteins are type of integral proteins meaning that they are permanently attached to biological membrane. Transmembrane proteins span through the plasma membrane so most of their structure is hydrophobic.

The major role of these proteins is transport role: they act as a channels that undergo conformational change in order to  move a substance through the membrane.

Classified by their structure they can be put into two types: alpha helical and beta barrels.

your answer is "the 1950's"