What's the secret ingredient in beef jerky? I've been making beef jerky using various marinade recipes I found on the WWW. The results have been mixed, but always lack that one distinct flavor that seems so prevalent in store-bought jerky. If I had to describe it, I'd say "protein-y". It's not a spice I recognize, and doesn't seem to be present in any combination of soy sauce, Worcestershire, etc.

Recently I found an old seasoning packet that came with my dehydrator, took a little taste, and recognized that same flavor immediately. Unfortunately there is no ingredient list printed on the packet. Where is that flavor is coming from?
"Proteiny" usually indicates "umami", the so-called "fifth flavor" after sweet, sour, bitter, and salty.  It's triggered by glutamates.  Common sources of umami include beef, mushrooms, some seaweeds (most especially nori), rinds of some hard cheeses (most especially pecorino romano) and tomato paste.  I wouldn't be at all surprised if the store-bought jerkies you are comparing it to include monosodium glutamate, a common food additive that is essentially concentrated umami flavor.