Intracellular protein traffic

The billion or so protein molecules of an average mammalian cell are constantly being renewed - that is, degraded and resynthesized, on average once every month. Each cell contains more than 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. Essentially, proteins are synthesized in only one of the cellular compartments, the cytoplasm (although a few are synthesized in the mitoplasm of mitochondria and the chloroplasm of chloroplasts). How are newly synthesized proteins directed to their proper membranes and compartments? And how do these relatively large molecules traverse the compartmental membranes without destroying the essential gradients of small molecules and ions that exist across each of these membranes? How are membrane proteins integrated into membranes such that each member of a given species of membrane protein exhibits precisely the same domains on the cis side and the trans side of the membrane?