Does marinating break down proteins the way brining does?
Brining does not technically "break down" proteins in meat. At least not the way marinating does.

Brining actually involves osmosis which carries salt and sugar inside the cell walls. This denatures the proteins causing them to unravel and interact with one another. This forms a matrix which traps moisture in the meat.

Marinating on the other hand actually does "break down" the proteins using acidity. The acid literally does consume the proteins and break down the texture of the meat. If the marinade has salt in it, then osmosis will occur as well and the marinade is also serving as a brine.

So, to answer your question: Yes, a salty marinade will brine your meat, but the protein breakdown that occurs is due to the acidity of the marinade.