This is amazing how many times I was asked: where do you get your protein? Many people seem to think that there is no protein in fruit. Let's look into it. 

1. How much protein one needs? ↓
2. How much protein is in fruit and seeds? ↓
3. Is that the right protein? ↓

How much protein do we need?

Let's start from the very beginning. Children require extra protein for growth. Mother’s milk, the essential food for babies during their fastest growth period, averages just 7% of its calories from protein. This allows an infant to grow by 4.25 kg (9.4 pounds) in 6 months, doubling the body weight. Adults do not need to grow that much, they need to rebuild and repair their bodies, which takes much less essential parts of proteins.

http://www.medindia.net/patients/calculators/ideal_weight_infant.asp

There is 1.1 g per 100 ml total protein in breast milk.

http://www.unu.edu/unupress/food/8F174e/8F174E04.htm

It is nearly the same is in many juicy fruits, by the way (~ 1 g per 100 g). For example, guavas - 2.6 g, mulberries - 1.4 g, apricots and blackberries - 1.4 g, raspberries - 1.2 g, cherries and nectarines - 1.1 g, oranges - 1 g, melons - 0.8 g, watermelon - 0.6 g (all proteins is per 100 g of the edible part of fresh fruits).

Other fruitarian foods like seeds can be much higher in protein: pumpkin seeds - 28 g, peanuts - 24 g, almonds - 21 g, sunflower seeds - 19 g, walnuts and hazelnuts - 15 g, pecans - 9 g, chestnuts - 4 g (all in grams per 100 g or ~ 1/5 pound of dry edible part).

The research tells us that exclusively breastfed babies take in an average of 25 oz (750 mL) per day between the ages of 1 month and 6 months. Different babies take in different amounts of milk; a typical range of milk intakes is 19-30 oz per day (570-900 mL per day).

http://www.kellymom.com/bf/pumping/milkcalc.html

Thus, a baby consumes on average 8.25 g of protein a day for the first half year of life - ~ 1.5 kg of protein total in this period (182.5days), adding a few kilograms (4.25) of high quality body tissues (not just fat).

The science of nutrition is in its infancy. The debate on how much proteins adult body needs for optimum health is still going on.

The difficulty of establishing human protein needs arises from the great complexity of food digestion and utilization. The principal method used in studies and experiments is based on the measurement of intake and excretion of nitrogen (specific component of proteins, making 16% of its weight). The assumption is that a balance between nitrogen input and output - adjusted for known interfering factors - indicates the level of protein intake needed to maintain protein level required for body functions. Thus the amount of protein needed for achieving protein balance would equal the minimum safe protein intake for adults (and children require extra protein for growth).

However, limitations and incompleteness of (many) conducted tests, coupled with hard to assess changes in protein metabolism due to a number of known and unknown variables has prevented, thus far, arriving to a unanimous result and agreement on this subject.

Officially recommended protein intake can vary significantly from one country to another, much more so than experimentally established protein needs of different human groups, or similar groups in different world regions. For an average moderately active adult female or male, requiring 2400-2800 calories a day, minimum safe protein intake is under 10% of total calories.

The recommended allowance of daily protein intake is ~ 0.75g per 1 kilogram of body weight for a lean person. For me (~ 50 kg), it would be approximately 36 g a day. Fruitarian food (fruits and seeds) give up to 250 g protein per 1 kg. Even with only 7 g per 1 kg in some very watery fruits, I can eat 5 kg of this fruits and get the estimated/recommended amount. With with 1 g per 100g (10 g per 1 kg) in mixed common fresh juicy fruit only, I would follow the recommendation by eating 3.5 kg (~ 7 pounds). 150 g of peanuts only would cover it too. 

How much protein needed in a person's daily diet is determined in large part by overall energy intake, as well as by the body's need for nitrogen and essential amino acids. Physical activity and exertion as well as enhanced muscular mass increase the need for protein. Requirements are also greater during childhood for growth and development, during pregnancy or when breast-feeding in order to nourish a baby, or when the body needs to recover from malnutrition or trauma or after an operation.

World Health Organization, Food and Agriculture Organization of the United Nations , United Nations University (2007).
"Protein and amino acid requirements in human nutrition". 

How much protein is in plant foods?

Let's take a case where a person eats primarily juicy fruit: around 90% of total calorie intake, and compare the percentage of protein in it to the recommended 10%.

It the table below you see common fruit and percentage of calories from protein in them (counted on 100g):

You can observe that only in a few fruits the number is lower than 5, so one can get 5-11% of protein of total caloric intake, satisfying their energy needs with a variety of such fruits all the time.

Here are some more dense fruitarian foods (fruits, nuts and seeds - one does not need to destroy the plant to obtain them). Just a few examples:

USDA's National Nutrient Database for Standard Reference 

I genuinely wonder why this information is not evaluable all over the internet - I needed to do the calculations myself. Despite the fact that this method of counting is used in most works of nutritionist, there are no databases I could find easily with these numbers.

Is that the right protein?

We do not use complete proteins, we need only their parts to build our own unique proteins, thousands of them. Amino acids are the building blocks of protein, there are 22 of them total, and 14 we can surely synthesize ourselves.

Proteins are harder for the body to use for energy, they normally satisfy less than 10% of its caloric needs. Your organism will burn proteins only when its preferred energy sources are not available (carbohydrates and fats).

These amino acids can abundantly be found in plant foods, where the animals get them. The amino acid profile of staple foods is rarely if ever a limiting factor in human nutrition. The only common food that represents an "incomplete" protein is gelatin.

Proteins from plant sources contain adequate amounts of all of the amino acids that are essential in human nutrition. Our body gets all but 1/6 of the protein it needs from recycling old body tissue. This 1/6 must come from essential amino acids we eat.

Amino acids

Eight amino acids are generally regarded as essential for humans: phenylalanine, valine, threonine, tryptophan, isoleucine, methionine, leucine, and lysine. The distinction between essential and non-essential amino acids is somewhat unclear, as some amino acids can be produced from others.

Recommended daily amounts

The following table lists the World Health Organization (WHO) recommended daily amounts currently in use for essential amino acids in adult humans and amounts in common juicy fruit, which are higher than half of healthy adults estimated to have (using myself as an example, my weight is normally around 50 kg):

Continuing the table:

Let us compare this approximate target values with the actual content of amino acids per 1 kilogram (~2 pounds) of various basic fruits (bold number indicates higher amounts, red and bold - sufficient):

Amino acids in fruits

Comments for the table of amino acids in fruits and seeds

In simple words, you would need to eat, for example, roughly over 2 kg (~ 4 lb) of apricots and 1 kg (~2 pounds) of blueberries to ensure that your body can build all protein it needs from  provided amino-acids. This is if you want to consume juicy fruit only.

Amount of most essential amino acids is significantly higher in seeds and beans, though peas, legumes in general, and some nuts are botanically fruits. Thus, adding some of them to a diet seem to be a good idea. 

It looks like theoretically a person can cover the daily need with .5 kg (1 pound) of fresh green peas or with 160 g of peanuts only. However, different proteins have different levels of biological availability to the human body. Many methods have been introduced to measure protein utilization and retention rates in humans, but none of them are fully reliable.

Protein Digestibility Corrected Amino Acid Score (PDCAAS) is a method of evaluating the protein quality based on both the amino acid requirements of humans and their ability to digest it. A PDCAAS value of 1 is the highest, and 0 the lowest. The PDCAAS measures the quality of a protein based on the amino acid requirements (adjusted for digestibility) of a 2- to 5-year-old child (considered the most nutritionally-demanding age group). The PDCAAS takes no account of where the proteins have been digested.

Ratings of some groups of plant food: soy protein - 1, soy beans - 0.91, chickpeas - 0.78, black beans - 0.75, vegetables - 0.73, all legumes - ~ 0.7, peanuts - 0.52, wheat gluten - 0.25, fresh fruits - 0.64 but dry fruits - 0.48, cereals - 0.59 (to compare to animal protein sources: egg - 1, beef - 0.92). 

Combination

Eating various plant foods in combination can provide a protein of higher biological value. However, vegetarians do not need to intentionally combine different foods for this purpose.

Grain protein has a PDCAAS of about 0.4 to 0.5, limited by lysine. On the other hand, it contains more than enough methionine. White bean protein (and that of many other pulses) has a PDCAAS of 0.6 to 0.7, limited by methionine, and contains more than enough lysine. When both are eaten in roughly equal quantities in a diet, the PDCAAS of the combined constituent is 1.0, because each constituent's protein is complemented by the other.

By eating a variety of fruits it is possible to get a good score protein.

A Case of Protein Deficiency

The amino acids that are essential in the human diet were established in a series of experiments led by William Cumming Rose. The experiments involved elemental diets to healthy male graduate students. These diets consisted of cornstarch, sucrose, butterfat without protein, corn oil, inorganic salts, the known vitamins, a large brown "candy" made of liver extract flavored with peppermint oil (to supply any unknown vitamins), and mixtures of highly purified individual amino acids. 

The main outcome measure was nitrogen balance. Rose noted that the symptoms of nervousness, exhaustion, and dizziness were encountered to a greater or lesser extent whenever human subjects were deprived of an essential amino acid.

Rose WC, Haines WJ, Warner DT. The amino acid requirements of man. III.
The role of isoleucine; additional evidence concerning histidine. J Biol Chem. 1951

If they would put me on such a weird diet and even add all essential amino acids on top of that, I would likely feel nervous and dizzy as well :)

But still, it is good to keep in mind that we may need one or another extra type of these building blocks for a huge variety of proteins we need to construct in our bodies to function properly.