All Known Porins Are _____ Proteins Composed of _____ $\alpha$ -Helices
B) Dimeric;

All known porins are _____ proteins composed of _____.

A) monomeric; seven transmembrane $\alpha$ -helices
B) dimeric; $\beta$ -barrels
C) dimeric; three transmembrane $\alpha$ -helices
D) trimeric; $\beta$ -barrels
E) trimeric; four transmembrane $\alpha$ -helices