Each cell contains about a dozen compartments enclosed by a membrane. Each of the membranes and the compartments they enclose are composed of distinct proteins, and both the compartments and their surrounding membranes carry out distinct functions.
Membranes are impermeable to proteins.
A signal sequence is present in each newly synthesized protein to be targeted to one of the cell's compartments. The signal sequence is first recognized by a soluble signal recognition factor that in turn binds to a cognate receptor in the target membrane. After translocation, the channel closes, to be gated open again by the cognate signal sequence of the next protein to be translocated, and so on. Of particular interest is that PCCs are also used for the integration of membrane proteins into the lipid bilayer. Hence the PCC, which is able to respond to sequence information in the translocating chain, is more than merely a passive conduit.