|Tuna is a decent protein source!|
First of which, and this is the chemist in me being peadantic, protein is not a nutrient. Proteins are the major building blocks of life - they're what you and everything you eat is made from on a chemical level - but the individual proteins are too specific. Instead, your body breaks them into the 22 amino acids that are their component parts, and then uses those amino acids to build up the proteins it uses to do human body things.
This is where the opportunity to optimize comes in. Instead of consuming the recommended amount of protein: .8 grams of fat per kilogram of body weight - I did the conversion because I couldn't stand that ludicrous formula from last time - consume the best amino acids. The amount of protein you need is going to go up with your activity level and with conditions like pregnancy and lactation. Obviously you should consult a medical expert over such things.
Of the 22 Acids there are several that are essential in adults: Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, and Valine. Google's spell-check insists that none of those things exist, but I assure you, they are real amino acids. Who, while your body can use all amino acids, prioritize these nine.
Histidine is used by the body for a number of things and is found mostly in meats (beef is richest, but all the farm animals contain it). The highest concentrations are seen in game meats for reasons passing understanding. It is also found in legumes and nuts, particularly in soy, so yes, with the right diet, there's no reason you couldn't be vegan and get enough histidine without supplementation.
Isoleucine is chiefly found in eggs, soy protein, and seaweed. Having said that, it is stockpiled in animals (which can't produce it naturally) so turkey, lamb, cheese, and fish are all good sources as well. Leucine, a chemical relative, shares the trate that it cannot be made by animals: you need to find that in your diet. While meats are the best sources, it's also in your legumes.
Lysine helps with your body's energy production and is chiefly found in fish, though it is also present in beef and, to a lesser extent, your legumes and cheeses.
... In point of fact, all of the essential amino acids can be obtained from either plant or animal sources, though you will have difficulties obtaining Methionine (used for some protein construction inside the body) and Phenylalanine (present in milk and some neurotransmitters) if you are following a strictly-vegan diet.
As with all things, moderation is the key, and unless you're trying to get overly obsessive-compulsive with your amino acid balance, merely eating a variety of protein-bearing foods that total up to your daily intake every day will be enough to keep you running smoothly.